Compounds and infrared devices including stoichiometric semiconductor compounds of indium, thallium, and including at least one of arsenic and phosphorus

Abstract

A semiconductor layer of In₁-x Tl_x Q carried on a substrate forms an infrared device, where Q is selected from the group consisting essentially of As₁-y P_y and 0<x<1, 0<y<1.

Description

FIELD OF INVENTION

The present invention relates generally to components including indium and thallium microns micrometers and for separately detecting medium wavelength infrared energy having a cutoff wavelength of 5 microns micrometers. The structure illustrated in FIG. 1 can be operated as a photovoltaic or as a photoconductive detector, depending on the bias voltages to which electrodes thereof are connected.

In the embodiment of FIG. 1, indium phosphide (InP), semi-metallic bulk substrate 10 has deposited thereon, preferably by any of liquid phase epitaxy, metalorganic chemical vapor epitaxy, metalorganic molecular beam epitaxy or molecular beam epitaxy methods, a rugged semiconductor alloy layer of n type indium thallium phosphide 12 (In₁- x1Tl_x1 P In₁-x1 Tl_x1 P) having a zinc blende structure. InP substrate 10 is a good electronic material, i.e. has high carrier mobility, low dislocation density (similar to GaAs), has virtually no native point defects, easily has ohmic contacts and p-n junctions formed thereon, is easily doped, can easily be coated with a function passivant/insulator (SiO₂), and electronic devices formed thereon have good, consistent performance characteristics. The value of x for the compound of layer 12 is selected such that layer 12 absorbs long wavelength infrared energy preferably having a cutoff wavelength of 12 microns micrometers and is approximately lattice matched to InP substrate 10. Based on studies we have performed, lattice matching is about 1% and the 12 micron micrometers wavelength cutoff are attained with a value of x₁ =0.67, whereby layer 12 has a bandgap of about 0.1 eV. Layer 12 is doped with silicon to achieve n type conductivity.

Deposited on layer 12, also by any of the foregoing methods, is a further indium thallium phosphide semiconductor layer 14 (In₁-x2 Tl_x2 P). Based on studies we have conducted, with x₂ =0.57 rugged layer 14 absorbs midrange infrared energy having a cutoff wavelength of 5 microns micrometers (associated with a bandgap of 0.28 eV), while passing the long wavelength infrared energy that is absorbed by layer 12. The indium thallium phosphide compound of layer 14 is doped with any one of zinc, magnesium or beryllium to form a p type layer, whereby a p-n homojunction is formed at the intersection of layers 12 and 14.

Aluminum ohmic contacts 16, 18 and 20 are respectively formed on exposed upper surfaces of layers 12 and 14 and substrate 10. Electrodes 16, 18 and 20 are connected to suitable electronic circuits to bias the device into a photoconductive state or enable the device to operate in the photovoltaic mode. All remaining, exposed surfaces of substrate 10 and of layers 12 and 14 are covered with passivating silicon dioxide (SiOn) layer 20.

The structure of FIG. 1 can be modified to detect infrared energy having a single cut-off wavelength of 5 microns micrometers or 12 microns micrometers. To provide a cut-off wavelength of only 5 microns micrometers, layer 14 and electrode 16 associated therewith are eliminated and the device is arranged so the infrared energy is incident on layer 12. To provide a cut-off wavelength of 12 microns micrometers, layer 12 is replaced with a superlattice arrangement of In₁-x3 Tl_x3 P (where x₃ varies from 0.67 to 0.57) that is lattice matched to substrate 10.

In an actual staring infrared focal plane detector, many devices of the type illustrated in FIG. 1 are arranged in a matrix of rows and columns on InP substrate 10 on which are also deposited CMOS bias and readout transistors, as well as metal row and column strips and other components.

While the preferred configuration includes an InP substrate and one or more In₁-x Tl_x P layers, the substrate can also be bulk semimetallic InAs carrying a semiconductor zinc blende layer of In₁-x3 Tl_x3 As, where 0≦x₃ ≦1; for a 5 micron micrometers cut-off of the In₁-x3 Tl_x3 As layer, x₃ =0.15. The invention is not limited to the pseudobinary compounds In₁-x Tl_x1 P and In₁-x2 Tl_x2 As for layers 12 and 14, but can be expanded to include the generalized pseudotertiary compound In₁-x Tl_x1 P In₁-x4 Tl_x4 Q, where Q is selected from the group consisting essentially of As₁-y P_y, where 0<x₄ <1 and 0≦y≦1. For y=0 or y=1, we have the specialized cases of the pseudobinary compounds In₁-x3 Tl_x3 As and In₁-x3 Tl_x3 P, respectively. For 0<y<1, we have the above-noted generalized pseudotertiary compound which our studies show can detect and emit infrared energy to tailored wavelengths in the spectra of interest. It is to be understood that the substrates are not limited to the preferred compounds of InP and InAs but that the substrate can be formed of other materials, particularly silicon or gallium arsenide in semiconductor form. The In₁-x Tl_x Q layer carried by such a substrate is physically connected to the substrate by an appropriate superlattice structure.

Based on the studies we have performed, the bandgap energies (hence the cut-off wavelengths) of In₁-x Tl_x P and In₁-x Tl_x As (shown in FIG. 2 by plotted lines 22 and 24, respectively) in accordance with this invention and the prior art compound Hg_x Cd₁-x Te (shown by plotted lines 26) are illustrated as a function of the value of x in the interval 0≦x≦1. From FIG. 2, any desired band gap, hence cut-off wavelength, can be attained by proper selection of the value of x. The higher electron mobilities of the compounds of the present invention relative to the mobility of the prior art HgCdTe at temperatures approaching room temperature are clearly shown in FIG. 3. FIG. 3 includes plots based on our studies of electron mobility (in 10⁵ cm² /V.sec 10⁵ cm² /V·s) vs. temperature of In₀.33 Tl.67 P (line 28), In.85 Tl.13 As (line 30) and Hg.78 Cd.22 Te (line 32), all p doped with zinc at 10¹4 /cm³ to have a band gap energy of 0.1 eV. Hence, our studies show that layers of the present invention do not require the extensive refrigeration structure required by the prior art.

The ruggedness, i.e. structural stability, of the zinc blende (four-fold coordination) In₁-x Tl_x P and In₁-x Tl_x As lattice structures of the present invention can be determined from the binding energy of the atoms of these compounds. Our studies have shown these compounds in zinc blende form to be light open structures having strong directional bonds relative to other compounds of the cations Al, Ga and In with the anions P, As and Sb in more closely packed NaCl (six-fold coordination) and CsCl (eight-fold coordination) structures. For TlSb, the NaCl and CsCl structures overtake the zinc blende structure, a manifestation of which is a very negative band gap of TlSb. This reversal in the ordering of the TlSb energy causes complications when attempts are made to grow the prior art In₁-x Tl_x Sb alloy. Our studies show that TlP and TlAs are stable relative to the more closely packed phases of the prior art compounds, whereby In₁-x Tl_x P and In₁-x Tl_x As are stable and can be produced without excessive problems.

Our studies show In.33 Tl.67 P has excellent long wavelength infrared properties relative to the prior art Hg.78 Cd.22 Te because inter alia:

1. its 5.96 Å 596 pm (picometer) lattice constant closely matches the 5.83 Å 583 pm (picometer) lattice constant of the InP substrate on which it is deposited so the In.33 Tl.67 P liquidus and solidus phase diagrams have simple lens shapes;

2. the cohesive energy per atom (2.56 eV/atom) of TlP is 58% greater than that of HgTe (1.62 eV/atom);

3. TlP is a semimetal having a band gap of -0.27 eV, about the same as HgTe (-0.3 eV);

4. its band gap concentration variation (dEg/dx) of 1.42 is 16% smaller than Hg.78 Cd.22 Te;

5. its elastic constants are about 33% greater than those of Hg.78 Cd.22 Te;

6. the temperature variation of the band gap (dEg/dT) near 77°K 77K is small (about -0.05 meV/°K. meV/K), approximately 15% that of Hg.78 Cd.22 Te (about -0.36 meV/°K. meV/K); because of the low value of dEg/dT for In.33 Tl.67 P, design of circuits including that compound for variable temperature operation is greatly simplified and spatial variations in pixel performance of detector elements in a large matrix array due to temperature gradients within the array are virtually eliminated;

7. its electron effective mass is 0.008, equal virtually to that of Hg.78 Cd.22 Te; and

8. its hole effective mass is 0.37, 43% smaller than the 0.65 hole effective mass of Hg.78 Cd.22 Te, resulting in considerably higher hole mobility and substantially longer electron Auger recombination lifetimes for In.33 Tl.67 P.

While there have been described and illustrated several specific embodiments of the invention, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, the invention can be used to form a solar cell having a cut-off wavelength such that a very large portion of the infrared spectrum is converted by photovoltaic action into electrical energy; in such an instance a layer of In.24 Tl.76 P is formed on an InP substrate. The compounds of the invention can also be used to form layers of infrared emitters in combination with the usual other structures of such emitters.

Claims

1. An infrared detector or emitter device comprising a substrate, and a semiconductor layer of In₁-x Tl_x Q carried by the substrate, where Q is selected from the group consisting essentially of As₁-y P_y and 0<x<0<x<1, 0≦y≦1.

2. The device of claim 1 where y=1.

3. The device of claim 2 where x=0.67.

4. The device of claim 2 where x=0.57.

5. The device of claim 1 where x=0.24.

6. The device of claim 2 wherein the layer is formed on the substrate and the substrate portion on which the layer is formed consists essentially of InP.

7. The device of claim 6 wherein the layer is doped to have a first conductivity polarization.

8. The device of claim 7 wherein another layer of In₁-x Tl_x P having the second conductivity polarization is formed on the layer having the first polarization to form a p-n homojunction.

9. The device of claim 1 wherein y=0.

10. The device of claim 9 wherein x=0.15.

11. The device of claim 10 wherein the layer is formed on the substrate and the substrate portion on which the layer is formed consists essentially of InAs.

12. The device of claim 1 wherein the layer is doped to have a first conductivity polarization, and a second layer having substantially the same compound as the layer having the first conductivity polarization contacting the first conductivity polarization layer to form a p-n homojunction, the second layer being doped to have a second conductivity polarization.

13. The device of claim 1 wherein the layer is formed on the substrate and the substrate portion on which the layer is formed consists essentially of InQ, the layer and substrate having substantially the same lattice constants.

14. The device of claim 13 where y=1.

15. The device of claim 13 where y=0.

16. The device of claim 13 where 0<y<1.

17. The device of claim 1 where 0<y<1.

18. The device of claim 1 wherein the device is a detector and the substrate includes a second layer of In₁-z Tl_z Q, where z is less than x, the second layer being positioned above the In₁-x Tl_x Q layer so certain optical radiation wavelengths incident on the second layer pass through the second layer and are absorbed by the In₁-x Tl_x Q layer and other optical radiation wavelengths incident on the second layer are absorbed thereby.

19. The device of claim 18 where y=1, x=0.67, z=0.57.

20. The device of claim 1 where y=1, x=0.24.

21. The device of claim 1 further including an ohmic contact on the layer.

22. In In₁-x Tl_x Q, where Q is selected from the group consisting essentially of As₁-y P_y and 0<x<1, 0≦y≦1.

23. The composition of claim 22 where y=0.

24. The composition of claim 22 where y=1.

25. The composition of claim 22 where 0<y<1.

26. A stoichiometric semiconductor compound comprising the elements In, Tl, and including at least one of the elements of As and P, wherein the compound is stoichiometric with respect to at least three of said elements.

27. The stoichiometric semiconductor compound of claim 26 wherein the compound is stoichiometric with respect to four elements including at least three of said four elements. 28. The stoichiometric semiconductor compound of claim 27 wherein the compound includes dopants.

29. The stoichiometric semiconductor compound of claim 26 wherein the compound is an alloy including at least three of said elements

in a zinc blende structure. 30. The stoichiometric semiconductor compound of claim 26 wherein the compound is stoichiometric with respect to four elements, including said four elements.

31. An infrared detector or emitter device comprising a substrate, and a semiconductor layer carried by the substrate, the layer including a stoichiometric semiconductor compound comprising the elements In, Tl, and including at least one of the elements of As and P, wherein the compound is stoichiometric with respect to at least three of said

elements carried by the substrate. 32. The infrared detector or emitter device of claim 31 wherein the compound is stoichiometric with respect to four elements, including at least three of said four elements. 33. The infrared detector or emitter device of claim 32 wherein the compound includes dopants.

. The infrared detector or emitter device of claim 31 wherein the compound is stoichiometric with respect to four elements, including said four elements.