Multi-Color Coincident Infrared Detector

This invention relates generally to radiation sensing detectors and more particularly to detectors producing plural electrical outputs in response to impinging radiation at distinctly different wavelengths.

The use of epitaxial semiconductors as infrared detectors in a multi-color imaging system, is generally well known. In such prior detector arrangements, photovoltages are developed across separate junctions respectively formed on the semiconductors having different bandgaps and on which separate optically active surfaces are established in order to achieve multi-color operation. Since each junction responds to only a spatial portion of the incident optical input, there is a loss of signal content. Further, when such detectors are arranged in an array configuration signal loss is especially severe because of non-coincident radiation focussed on separate areas at different times.

The use of two radiation absorbing layers of a photosensing device that are stacked in alignment with a common axis of coincident impinging radiation to respectively absorb different components of such radiation at different wavelengths, is disclosed in U.S. Pat. No. 4,651,001 to Harada et al. One of such layers is made of amorphous silicon according to the Harada et al. patent to sense only a visible wavelength component of input imaging light while the other radiation absorbing layer formed on a substrate is made of a photovoltaic platinum-silicon compound to absorb only an infrared component of light within a totally different spectral range. Obviously, the arrangement disclosed in the Harada et al. patent is incapable of, and not designed to, separately sense mutually exclusive components of radiation at different wavelengths within the same spectral range.

In accordance with the present invention, radiation absorbing layers are epitaxially grown in stacked relation on a substrate in alignment with a common axis of coincident input radiation impinging on an optically active surface area, all of such layers being made of photovoltaic materials having different bandgap energy properties that are so interrelated to effect radiation absorption at separate and distinct wavelengths within a common spectral range. The radiation absorbing layers are spaced from each other by semi-insulating layers to achieve electrical separation between radiation responsive signal outputs at the different wavelengths, through separate diode junctions respectively associated with the radiation absorbing layers in a heterojunction arrangement for multi-color signal processing operation.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a side view of a heterojunction type of infrared detector device, in accordance with one embodiment of the present invention, together with a schematic block diagram of a signal processing system associated therewith.

FIG. 2 is an energy band diagram for the detector device depicted in FIG. 1.

FIG. 3 is a partial side view of an alternative hetorojunction device in accordance with another embodiment of the invention.

Referring now to the drawing in detail, FIG. 1 depicts a heterojunction device, generally referred to by reference numeral 10, which may form part of a photodetector array operatively connected to a signal processing system 12, a portion of which is shown. A focussed beam of radiation 14 from some imaging system for example, is diagrammed in FIG. 1 in alignment with a coincident axis 16 for impingement on a common optically active surface 18 of device 10. In the illustrated embodiment, the surface 18 is centrally intersected by the axis 16.

The embodiment of device 10 illustrated in FIG. 1 includes three semiconductor layers 22, 24 and 26 stacked in alignment with axis 16 on a substrate 28. Each of the layers 22, 24 and 26 is made of a compatible photovoltaic material having interrelated, radiation absorbing properties arranged to separately absorb radiation at distinctly different wavelengths within a broad common spectral range, such as the infrared spectrum. The layers are furthermore spaced from each other by semi-insulating spacers 30 and 32 preferably made of layers of semiconductor material exhibiting bandgap energy levels higher than those of the radiation absorbing layers 22, 24 and 26 by 0.05 eV or more. The radiation absorbing layers furthermore exhibit bandgap energy levels decreasing in magnitude in a direction away from the substrate 28 on which the layer 26 is deposited. Accordingly, the layer 26 forms a wide bandgap region 34 as compared to a narrow bandgap region 36 and an intermediate bandgap region 38 as diagrammed in FIG. 2, said bandgap regions being respectively occupied by layers 22 and 24. A metal contact 20, is interfaced on the layer 22 furthermost from the substrate 28 as shown in FIG. 1.

As shown in the energy band diagram of FIG. 2 corresponding to the device 10, the wide bandgap region 34 is defined between a conduction band edge 40 and a valence band edge 42 and extends from the interface 44 between substrate 28 and layer 26 to its interface with semi-insulating spacer 32 at which a rapid step-like increase in bandgap energy occurs to energy band region 46 establishing a higher bandgap energy region for the spacer 32 between the wide bandgap region 34 and the intermediate bandgap region 38 occupied by layer 24. Similarly, an increase in bandgap energy in energy band region 48 occurs between the intermediate and narrow bandgap regions 38 and 36, as diagrammed in FIG. 2. The narrow bandgap region 36 is terminated, as shown, at the interface 50 between layer 22 and the metal contact 20.

In view of the semi-insulating properties exhibited by the high bandgap spacers 30 and 32, the signal outputs extracted from the radiation absorbing layers 22, 24 and 26 are electrically separated. Thus, the signal outputs at respectively different wavelengths, corresponding for example to three different imaging colors, may be extracted by electrical leads 52, 54 and 56 from the layers 22, 24 and 26 through ohmic contact connectors 58, 60 and 62 to separate signal receivers 64, 66 and 68 as diagrammed in FIG. 1. The signal receivers are electrically connected in common through lead 70 to the diode junction establishing contact 20 of device 10 in order to supply individual and separated color signals to a multi-color signal imaging processor 72 of system 12. In order to facilitate better separation of radiation generated charge carriers producing the signal outputs from the radiation absorbing layers, the action of an electric field is utilized. Toward that end, a reverse bias between metal contact 20 and layer 26 is applied by means of field generator 74 as also diagrammed in FIG. 1.

The photovoltaic materials utilized for the semiconductor layers 22, 24 and 26 preferably belong to a class of lattice-matched groups, such as PbSe, PbSnSe and PbCdSe. Similar chalcogenides including in addition to the aforementioned selenides sulfides and tellurides may also be utilized. The semiconductor layers are deposited on each other as epitaxially grown crystals in stacked relation on the substrate by available hetero-epitaxy growth technology. Such method of construction was found to enhance performance in terms of the ability to discriminate between wavelengths of the same spectrum. Also, for proper performance band edge alignment is inherent in the detector arrangement described as evident from the energy band diagram of FIG. 2.

The multi-wavelength signal output responses from the radiation absorbing layers may alternatively be obtained through plural p-n junctions, where corresponding radiation absorbing layers 22', 24', and 26' are all of one p or n type. In such case, the metal contact 20 is deleted in favor of contact connections to the semi-insulating spacers 30' and 32' of the other type (p or n), in order to establish diode junctions between different pairs of radiation absorbing layers and adjacent spacing layers through leads 52', 54', 56', 71' and 72' connected to signal receivers 64', 66 and 68' as shown in FIG. 3.

Numerous other modifications and variations of the present invention are possible in light of the foregoing teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.