Abstract:
In this invention, there is disclosed a new photodiode which comprises a semimetal/semiconductor junction. Distortion of the bands associated with this type of junction result in current flow when an electron hole pair is formed by, for example, a photon impinging upon the junction. The photodiode operates in the photovoltaic mode. However, rather than relying on impurity doping to fabricate a p-n junction, the semimetallic nature of HgTe and its large conduction band offsets with Hg 1-x  Cd x  Te are used to create a rectifying Schottky like structure.

Description:
TECHNICAL FIELD 
     This invention relates generally to a photodiode and more particularly to a photodiode which can be used for infrared applications. 
     BACKGROUND OF THE INVENTION 
     In recent years the alloy, mercury cadmium telluride Hg 1-x  Cd x  Te has been recognized as the most important material for mid-and long-wavelength infrared application. In the publication, in &#34;J. Vac Sci Technol A6(4)&#34;, July/August 1988--the article entitled &#34;Mercury cadmium telluride junctions grown in situ by molecular-beam epitaxy&#34;--pp. 2623-26 n-isotype mercury cadmium telluride heterojunctions were made in situ by molecular beam epitaxy. The cadmium composition of both sides were doped with indium. In the &#34;Concluding&#34; paragraph of the publication, the statement is made that &#34;The control of doping is also a very difficult task to achieve in this II-VI ternary alloy.&#34; 
     SUMMARY OF THE INVENTION 
     In this invention, there is disclosed a new photodiode which comprises a semimetal/semiconductor junction. Distortion of the bands associated with this type of junction result in current flow when an electron hole pair is formed by, for example, a photon impinging upon the junction. The photodiode operates in the photovoltaic mode. However, rather than relying on impurity doping to fabricate a p-n junction, the semimetallic nature of HgTe and its large conduction band offsets with Hg 1-x  Cd x  Te are used to create a rectifying schottky like structure. 
     The advantages of this structure are that doping is not required; the material is n-n, it is not p-n; and, the mercury cadmium telluride alloy system has a broad wavelength sensitivity. In addition, the materials are lattice matched, and, therefore, can be grown epitaxially. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the Drawing: 
     FIG. 1 is an illustrative embodiment of a back illuminated photodiode in accordance with the principles of the invention; 
     FIG. 2 is a schematic band diagram of an n-Hg 0 .54 Cd 0 .46 Te/n-HgTe isotype heterojunction; 
     FIG. 3 is a plot of the current-voltage characteristic of a photodiode in accordance with the principles of the invention; and 
     FIG. 4 is a plot of the spectral response and quantum efficiency of a photodiode in accordance with the principles of the invention operating at room temperature when unbiased and with a reverse bias of 250 mV. 
     FIG. 5 is a schematic diagram of a photodiode in accordance with the principles of the invention for detecting more than one wavelength; and 
     FIG. 6 is a plot of the responsivities of the device of FIG. 5. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there illustrated a cross-sectional view of a mercury cadmium telluride Hg 1-x  Cd x  Te all epitaxial semiconductor-semimetal Schottky type of photodiode in accordance with the principles of the invention. For convenience, the photodiode in FIG. 1 is not drawn to scale. In the description which follows the term &#34;semimetal&#34; is used. The term &#34;semimetal&#34; means a material having a conduction band which meets the valence band or, stated differently, the bandgap is zero or negative. 
     The photodiode is adapted to be back illuminated and comprises substrate layer 10 of GaAs cut and aligned for a (100) orientation. On the (100) surface of layer 10 there is grown, by known molecular beam epitaxy, a buffer layer 12 of ZnTe having a thickness of approximately 0.3 μm. A second buffer layer 14 is grown onto layer 12, the second buffer layer being of semi-insulating CdTe having a thickness of approximately 1.9 μm thick and being grown by molecular beam epitaxy. 
     An epitaxial semiconductor/semimetal Schottky type of photodiode is grown on the second buffer layer 14 by molecular beam epitaxy. Specifically, first, a 2000 Å thick HgTe layer 16 is grown on the CdTe layer and then a 2.25 μm thick Hg 0 .54 Cd 0 .46 Te layer 18 is grown on the HgTe layer. The substrate temperature was 300° C. for each of the buffer layers 12 and 14, and 180° C. for the Hg containing layers 16, 18 (as grown by molecular beam epitaxy without intentional doping. 
     The HgTe layer 16 and the Hg 0 .54 Cd 0 .46 Te layer 18 are each n-type having carrier concentrations, as determined from Hall measurements of single layers grown under similar conditions directly on semi-insulating CdTe buffer layers, of approximately 3×10 17  cm 3  and 5×10 15  cm -3  respectively. 
     A diode is formed by electroplating a Au contact 20 onto the surface of the Hg 0 .54 Cd 0 .46 Te layer 18, etching the mesa by using a mask of low melting temperature wax, and then plating a second Au contact 22 onto the exposed HgTe layer 16. The Hg 0 .54 Cd 0 .46 Te layer 18 can be etched with a 3-2-1 etch (3HF:2H 2  0 2  :1D.I.H 2  0) and the HgTe layer 16 serves as a stop-etch layer to automatically control the depth of the etch. The photodiode has an isotype heterojunction between n-type HgTe and n-type Hg 0 .54 Cd 0 .46 Te which can be more generally identified as Hg 1-x  Cd x  Te where 0&lt;×&lt;1. HgTe and Hg rich compositions of Hg 1-x  Cd x  Te are n-type as grown by molecular beam epitaxy. As X approaches 1, doping with an n-type of impurity such as indium can be used. 
     The ability to grow abrupt heterojunctions in the Hg 1-x  Cd x  Te alloy has made it possible to determine the band offsets in the invention here described. It is now generally accepted that the Cd Te-HgTe valence band offset amounts to approximately 25% of the band gap difference. Because of the semimetallic nature of HgTe, the resulting conduction band offset is approximately 75% of the bandgap of CdTe. It is this large offset which provides the device with its unusual electrical properties. 
     For n-type Hg 1-x  Cd x  Te on HgTe, a rectifying barrier is formed in the conduction band of the wide bandgap material. Similarly, a smaller barrier occurs from n-isotype heterojunctions between any two compositions of Hg 1-x  Cd x  Te. For Hg 1-x  Cd x  Te on the semimetal HgTe, the barrier height is approximately 75% of the band-gap and, this barrier, with the associated depletion layer, displays characteristics similar to those of a Schottky diode. 
     Referring to FIG. 2, there is illustrated the schematic band diagram for an n-Hg 0 .54 Cd 0 .46 Te/n-HgTe isotype heterojunction. It is a fundamental property that, at the interface, the energy differences between the valence band of the HgTe and the valence band of the Hg 1-x  Cd x  Te is approximately 25% of the bandgap difference. But, the band gap of the HgTe is essentially zero or negative. Therefore, the discontinuity (or offset) at the interface between the conduction bands is a very large portion of the band gap difference. 
     Stated differently, the discontinuity of the valence band is approximately one-quarter of the difference of the band gaps. The discontinuity in conduction bands is the difference in band gaps minus the discontinuity of the valence band. Thus, the discontinuity of the conduction bands is approximately 75% of the difference in the band gaps. 
     Thus, far from the interface the two conduction bands are almost at the same energy; and, close to the interface they are separated by approximately 75% of the band gap. To obtain this, a large band bending is required close to the interface. This is similar to the band bending which occurs in a p-n junction or a normal Schottky diode. Briefly, what is required is a relatively large discontinuity in the conduction bands which ideally would be substantially equal to the difference of the band gaps. 
     Referring to FIG. 3 there is illustrated the current-voltage characteristics of a 40 μm diameter all epitaxial semiconductor/semimetal Schottky type of photodiode made in accordance with the principles of the invention. Looking at FIG. 3 it can be seen that there is good rectification: 130μA of dark current at a reverse bias of 0.5 V at room temperature. The current-voltage characteristic illustrated in FIG. 3 can be better understood if reference is also made to the schematic band diagram of FIG. 2. Briefly, the large barrier in the Hg 0 .54 Cd 0 .46 Te conduction band blocks current flow unless the Hg 0 .54 Cd 0 .46 Te side of the heterojunction is biased negative. But, because of the semi-metallic nature of HgTe, the n-Hg 0 .54 Cd 0 .46 Te/n-HgTe heterojunction behaves as a Schottky barrier. 
     Reverse bias of the epitaxial semiconductor/semimetal Schottky type diode corresponds to a positive bias of the contact of the n-type Hg 0 .54 Cd 0 .46 Te relative to that on the HgTe as is expected for the n:Hg 0 .54 Cd 0 .46 Te/n:HgTe isotype heterojunction. 
     The spectral response and quantum efficiency of the epitaxial semiconductor/semimetal Schottky type of diode were determined by comparison to a calibrated InSb detector using a lamp, monochromator and reflecting optics. The results are illustrated in FIG. 4 for the detector operating at room temperature when unbiased and with a reverse bias of 250 mV. It is to be noted that the depletion layer created by the Hg 0 .54 Cd 0 .46 Te/HgTe n-isotype heterojunction operates to separate photogenerated electron-hole pairs in the same manner as the depletion layer of a conventional p-n junction or Schottky photodiode. For operation at 80° K., the maximum quantum efficiencies were N=44% without bias and, with a reverse bias of 1.6 V (i d  500nA), N=44% . Considering a 30% reflection loss from the GaAs backface and the absorption loss in the HgTe layer, this efficiency represents an internal quantum efficiency which exceeds 90%. 
     The high quantum efficiencies noted above are representative of a structure which has not been optimized for layer thickness, carrier concentrations, passivation and/or other device characteristics. Modifications, such as reducing the thickness of the GaAs substrate and the HgTe layer that the incident light must traverse may result in higher quantum efficiencies; passivation may result in lower dark currents; and, higher quality Hg 0 .54 Cd 0 .46 Te with lower carrier concentrations should result in higher unbiased responsivities. 
     Referring to FIG. 5, there is illustrated another embodiment of the all epitaxial semi-metal/semiconductor Schottky type of photodiode structure integrated vertically to provide a multiwavelength or multicolor detector. Specifically, onto the (100) surfaces 60 of a substrate of GaAs therein grown, by molecular beam epitaxy, a buffer layer 62 of ZnTe. A second buffer layer 64 is grown by molecular beam epitaxy onto layer 62, the second buffer layer being of semi-insulating CdTe. An epitaxial semiconductor/semimetal Schottky type of photodiode is grown on the second buffer layer 64 by molecular beam epitaxy. Specifically, an n:Hg 1-x1  Cd x1  Te layer 66 is grown on the CdTe layer 64, then an n-HgTe layer 68 is grown on that layer and, thereafter, an n-Hg 1-x2  Te layer 70 is grown onto the last grown layer 68. The diode is formed by electroplating a first Au contact 72 onto the surface of the first grown n-Hg 1-x1  Cd x1  Te layer 66; a second Au contact 74 is electroplated onto the surface of the second grown n-HgTe layer 68; and, a third Au contact 76 is electroplated onto the surface of the last grown n:Hg 1-x2  Cd x2  Te layer 70. 
     Each n-type HgCdTe layer forms an all epitaxial semimetal/semiconductor photodiode with the n type HgTe layer. If X 2  &lt;X 1  the corresponding bandgap wavelengths λ b2  &gt;λ b1  and the structure is suitable for back illumination. 
     When the photodiode is back illuminated through the substrate, wavelengths between λ bs  the substrate bandgap wavelength, and λ b1  will be absorbed in the lower HgCdTe layer 66 giving rise to a photocurrent between contacts 72 and 74. Radiation of wavelengths between λ b1  and λ b2  are transmitted by the lower HgCdTe layer 66. Most of the light is not absorbed in the thin HgTe layer 68, but it is absorbed in the upper HgCdTe layer 70 to generate a photocurrent between contacts 74 and 76. 
     Referring to FIG. 6, there is illustrated the responsivity vs. wavelength relationship for the device of FIG. 5. In FIG. 6, PCI represents a plot of the photocurrent between contacts 72 and 74; and, PC2 represents a plot of the photocurrent between contacts 74 and 76. Thus, in the embodiment of FIG. 5, two discrete wavelengths or colors can be detected independently and simultaneously with the same structure. 
     Returning to FIG. 5, the device illustrated is designed to be back illuminated when x 2  &lt;x 1  and λ b2  &gt;λ 1 . If, however, the structure is to be front or top illuminated, then x 2  &gt;x 1  and λ b2  &lt;λ b1 . 
     Clearly, if desired, additional active layers interleaved with HgTe layers can be stacked vertically to detect additional wavelengths in either a front or back illuminated configuration. 
     Clearly, there is disclosed above, for the first time, a totally new all epitaxial semiconductor/semimetal Schottky type of photovoltaic detector having a novel band structure which exhibits rectification at room temperatures, displays high quantum efficiency photoresponse in the identified Hg 1-x  Cdx Te/HgTe n-isotype heterojunction, and is epitaxial and thus suitable for integration.