Strained layer superlattice focal plane array having a planar structure
View Patent ↗An infrared focal plane array (FPA) is disclosed which utilizes a strained-layer superlattice (SLS) formed of alternating layers of InAs and In x Ga 1-x Sb with 0≦x≦0.5 epitaxially grown on a GaSb substrate. The FPA avoids the use of a mesa structure to isolate each photodetector element and instead uses impurity-doped regions formed in or about each photodetector for electrical isolation. This results in a substantially-planar structure in which the SLS is unbroken across the entire width of a 2-D array of the photodetector elements which are capped with an epitaxially-grown passivation layer to reduce or eliminate surface recombination. The FPA has applications for use in the wavelength range of 3-25 μm.
1. A method for fabrication of an infrared focal plane array photodetector having a two-dimensional (2-D) array of photodetector elements, comprising the steps of:
epitaxially growing a plurality of III-V compound semiconductor layers on a gallium antimonide (GaSb) substrate, with the III-V compound semiconductor layers including:
a buffer layer;
a strained-layer superlattice comprising a plurality of alternating layers of indium arsenide (InAs) and indium gallium antimonide (In x Ga 1-x Sb) with an indium content x in the range 0≦x≦0.5;
and a contact layer;
electrically isolating each photodetector element from the remaining photodetector elements in the 2-D array of photodetector elements while maintaining the strained-layer superlattice contiguous across the entire 2-D array of photodetector elements by selectively doping a portion of the plurality of compound semiconductor layers within or about each photodetector element;
depositing an electrode above the contact layer to independently electrically contact each photodetector element in the 2-D array of photodetector elements; and
depositing another electrode proximate to the GaSb substrate to form a common electrical connection to each photodetector element in the 2-D array of photodetector elements.
2. The method of claim 1 further comprising epitaxially growing a passivation layer above the plurality of III-V compound semiconductor layers, and etching an opening through the passivation layer at the location of each photodetector element.
3. The method of claim 2 wherein the passivation layer comprises GaSb, aluminum gallium antimonide (AlGaSb) or aluminum gallium arsenide antimonide (AlGaAsSb).
4. The method of claim 1 wherein the GaSb substrate is doped with a first doping type, and the step of epitaxially growing the plurality of III-V compound semiconductor layers comprises doping the buffer layer with the first doping type, and doping the contact layer with a second doping type.
5. The method of claim 4 wherein the step of epitaxially growing the plurality of III-V compound semiconductor layers comprises doping a first-grown portion of the strained-layer superlattice with the first doping type, and doping a last-grown portion of the strained-layer superlattice with the second doping type.
6. The method of claim 1 wherein the step of epitaxially growing the plurality of III-V compound semiconductor layers comprises epitaxially growing the buffer layer with a composition of GaSb, AlGaSb or AlGaAsSb.
7. The method of claim 1 wherein the step of epitaxially growing the plurality of III-V compound semiconductor layers comprises epitaxially growing the contact layer with a composition of GaSb or InAs.
8. The method of claim 1 wherein the step of selectively doping the portion of the plurality of III-V compound semiconductor layers comprises ion implanting an impurity dopant into the portion of the plurality of III-V compound semiconductor layers.
9. The method of claim 8 further comprising a step for annealing the ion-implanted impurity dopant at a temperature of about 450° C. or less.
10. The method of claim 1 further comprising a step of thinning the GaSb substrate to a thickness of less than or equal to 200 microns.