Abstract:
Spectral shift between different wavelength spectra by restricted narrow bandgap absorption of incident radiation at one location on a semiconductor body, under electrical bias causing release of radiation at another emission location as a result of radiative electron-hole recombination. The semiconductor body is a graded bandgap establishing composition of two selected compounds alloyed to a variable, position-dependent degree between the respective radiation and emission locations at which the respective narrow and wide bandgap properties of the compounds prevail.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to real-time imaging of infrared signals utilizing semiconductor materials. 
     The detection of radiation limited to infrared wavelengths in a region of interest, such as 3 to 5μm emitted from relatively hot bodies and 8μm to 12μm emitted from bodies at ambient temperatures, by use of a narrow band gap semiconductor, is generally well known. Visible imaging of signals associated with such detected infrared radiation, however, has heretofore involved a considerable amount of electronic processing. 
     The conversion of infrared radiation into visible imaging light is disclosed, for example, in U.S. Pat. No. 4,914,296 to Reinhold et al. U.S. Pat. No. 4,157,926 to Schoolar relates, on the other hand, to the use of thin film semiconductor crystalline material for detection of infrared radiation. The design and selection of semiconductor materials for detection of radiation at infrared wavelengths is disclosed, by way of example, in U.S. Pat. Nos. 4,195,226, 4,691,107 and 4,885,620 to Robbins et al., Elliot et al., and Kemmer et al. 
     Methods and apparatus are also generally known in the art for the synthesis of semiconductors with desired characteristics. Such prior art apparatus include molecular beam epitaxy and vapor phase epitaxy devices enabling the growth of high quality semiconductor films and tandem positive ion accelerators by means of which the semiconductor film may be implanted with ions to obtain the desired electronic properties relating, for example, to electron mobility, carrier lifetime and band-gap gradient. 
     It is therefore an important object of the present invention to provide for more direct, portable and less costly real-time imaging of detected infrared radiation signals by upconversion to visible radiation involving spectral shift in wavelength of visible radiation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, radiation is detected and absorbed within a body of semiconductor material having position dependent concentrations of compounds alloyed to form a graded bandgap between radiation absorbing and emitting locations to convert incident radiation within the infrared spectrum into radiation released within a substantially different wavelength region, such as the visible spectrum, for signal imaging purposes. A body of the semiconductor material such as a thin film crystalline layer is synthesized from a judicious selection of the alloyed compounds having in common elements from a group consisting of tellurium, selenium and sulphur to exhibit a relatively narrow bandgap at the radiation absorbing location for restrictively accommodating photon absorption of the infrared incident radiation as well as to enable radiative electron-hole recombination under appropriate positive electrical bias for release of upconverted radiation at the emitting location where a wider bandgap prevails. 
     In certain embodiments, an infrared sensitive phosphor screen is coupled to the semiconductor body and responds to the upconverted radiation released at the emitting location to produce the visible wavelength radiation for signal target imaging purposes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     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 schematic cross-sectional view of an infrared sensing semiconductor device in accordance with one embodiment of the invention; and 
     FIG. 2 is an electron energy bandgap diagram graphically characterizing the electronic properties associated with the semiconductor device of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing in detail, FIG. 1 illustrates a semiconductor device, generally referred to by reference numerical 10, constructed in accordance with one embodiment of the invention. The device 10 comprises a semiconductor body 12 suitably supported on substrate 14 which is transparent for detection of infrared radiation, for example, by receiving incident radiation denoted by reference numeral 16. A suitable electrical bias is applied by voltage source 18 across the body 12 between surface locations 20 and 22 thereon to produce an upconverted emission 24 from location 22 at wavelengths smaller than those of the incident radiation 16 absorbed by the semiconductor body 12 at location 20. Thus, as shown in FIG. 1 the voltage source 18 applies a positive forward biasing potential to the body 12 at location 20. A phosphor screen 26 is coupled to the semiconductor body at location 22 as shown to emit visible radiation in response to emissions from location 22 at wavelength suitable for real-time signal target imaging purposes. 
     Absorption of incident radiation by the semiconductor body 12 is limited to infrared wavelengths within the region of interest by selection of a narrow bandgap material component from a group of mercury and lead compounds consisting of: HgTe, PbTe, PbSe and PbS. Such compounds respectively absorb radiation at peak response wavelengths of 40μ, 4μ, 3-4μ and 2-2.5μ to establish a narrow bandgap 28 at the incident absorption location 20 on semiconductor body 12, as denoted in FIG. 2. Such bandgap is furthermore established between conduction band edge 30 and valence band edge 32, characterizing properties resulting from the synthesis of the semiconductor body by selection of the aforementioned narrow bandgap material component alloyed with a bandgap opening group of cadmium compound components consisting of: CdTe, CdSe and CdS or any other compatible compounds having wider bandgaps 34. A crystalline semiconductor compound of Hg or Pb selected from the narrow bandgap group aforementioned may be produced by an expitaxial growth process and mixed with a compound of Cd from the aforementioned wide bandgap group by ion implantation in accordance with one embodiment of the invention to synthesize the graded bandgap semiconductor body 12 having a composition formed as a continuously varying position dependent alloy of the two selected semiconductor compound components. The composition of the semiconductor body is thus selected from a group consisting of: Hg 1-x  Cd x ,Te, Pb 1-x  Cd x  Te, Pb 1-x  Cd x  Se and Pb 1-x  Cd x  S, where x is position dependent degree of alloying varying between zero and one corresponding to continuous variation from the narrow bandgap 28 in one direction to the wider bandgap 34 as depicted in FIG. 2 in order to achieve the purposes of the invention. 
     It will become apparent from the foregoing description that the grading of the bandgap will depend on the selection of the narrow and wide bandgap compound components of the semiconductor body 12 as well as the position dependent degree of alloying X. Accordingly, at the incident radiation absorbing location 20, where x=0, the semiconductor composition (HgTe, PbTe, PbSe or PbS) corresponds to that of the narrow bandgap compound component. At the emission location 22 on the other hand, where x=1, the semiconductor composition (CdTe, CdSe or CdS) corresponds to that of the wide bandgap compound component. 
     As shown in FIG. 1, the semiconductor body 12 at the narrow bandgap absorbing location 20 is biased positively with respect to the wide bandgap emitting location 22 by means of an electric field produced by voltage source 18 so that photon absorption at location 20 generates an electron-hole pair with electron drift along conduction band 30 in a direction toward the incident radiation or positive electrode location 20 as depicted in FIG. 2. The photogenerated electrons disappear at the positive electrode and reappear at the negative electrode location 22. At the same time, hole drift in the opposite direction occurs along valence band 32 at a slower rate resulting in radiative electron-hole recombination at the emission location 22 from which upconverted radiation is released The foregoing shift Δ(hv) in photon energy is presently known to be expressed as: ##EQU1## where μ and τ are mobility and lifetime of the electron carriers, F is the intensity of the applied electric field and dEg/dx is the bandgap gradient Because of the foregoing relationship, generally known in the art, and the hereinbefore described synthesis of the semiconductor body 12, the desired operational aspects of the invention are realized. Where the alloy composition resulting from the selected compounds in the semiconductor body is: Hg 1-x  Cd x  Te, for example, the corresponding operating parameters have been determined from the foregoing relationship as: μ=6.3×10 3  cm 2  v -1  s -1 , τ=2.10 -6  S, dEg/dx=52eVcm -1 , F=lV/cm and Δ(hv)=0.65. Incident radiation 16 having a wavelength of 4μ, by way of example, would then be upconverted for a spectral shift to a wavelength of 1.2μ, readily detected by the infrared sensitive phosphor screen 26 associated with device 10. 
     A detector array of a plurality of devices 10 has the capability of providing real-time visible imaging with a resolution dependent on the incident surface distribution d density of such devices 10 in the detector array. Further, the device 10 as hereinbefore described may be modified in accordance with the present invention to act as an infrared radiation source by polarity reversal of the electric bias field applied to the body thereby downconverting visible radiation to infrared radiation emitted from location 20 at a desired frequency. 
     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.