Lattice matched photocathodes for extended wavelengths
A photocathode epitaxial structure. The photocathode epitaxial structure includes a binary compound substrate material. The photocathode epitaxial structure further includes an active device absorber layer forming a portion of a p-type device photocathode formed on the binary compound substrate material. The active device absorber layer comprising at least a quaternary or greater material structure configured to be lattice matched with the substrate material to reduce strain to allow charge carriers to go further in the active device absorber layer implemented in the photocathode of a nightvision system.
1. A photocathode epitaxial structure comprising:
a binary compound substrate material; and
an active device absorber layer forming a portion of a p-type device photocathode formed on the binary compound substrate material, the active device absorber layer comprising at least a quaternary or greater material structure configured to be lattice matched with the substrate material to reduce strain to allow charge carriers to go further in the active device absorber layer implemented in the photocathode of a nightvision system; and
a selective barrier layer deposited on the active device absorber layer configured to act as a barrier for thermally generated electrons while freely passing energetic photogenerated electrons, wherein a thickness of the selective barrier layer is less than or equal to 10 nm.
2. The photocathode epitaxial structure of claim 1 , wherein the substrate material is GaAs and the active device absorber layer is InGaAsNSb.
3. The photocathode epitaxial structure of claim 2 , further comprising an InGaP etch stop layer to prevent surface damage.
4. The photocathode epitaxial structure of claim 1 , wherein the substrate material is InP and the active device absorber layer is InGaAsP.
5. The photocathode epitaxial structure of claim 4 , further comprising an AlInAsP etch stop layer.
6. The photocathode epitaxial structure of claim 1 , wherein the active device absorber layer formed on the binary compound substrate material has a direct optical band gap of 1.4 to 0.7 eV at 300 Kelvin.
7. The photocathode epitaxial structure of claim 1 , wherein the active device absorber layer formed on the binary compound substrate material detects optical wavelengths up to at least 1064 nm.
8. The photocathode epitaxial structure of claim 1 , wherein the active device absorber layer formed on the binary compound substrate material detects optical wavelengths up to at least 1200 nm.
9. The photocathode epitaxial structure of claim 1 , wherein the active device absorber layer formed on the binary compound substrate material detects optical wavelengths up to at least 1550 nm.
10. The photocathode epitaxial structure of claim 1 , wherein the active device absorber layer formed on the binary compound substrate material is doped exponentially by p-type impurities with levels of doping increasing away from an interface between the active device absorber layer and the binary compound substrate material.
11. The photocathode epitaxial structure of claim 1 , wherein the selective barrier layer comprises a fully strained GaAs or InP layer between an etch stop layer and the active device absorber layer.
12. The photocathode epitaxial structure of claim 11 , further comprising a Cs—O layer on the fully strained layer for activation.
13. The photocathode epitaxial structure of claim 1 , further comprising a window layer on the active device absorber layer.
14. A method of forming a photocathode absorber, the method comprising:
on a binary compound substrate material, forming an active device absorber layer forming a portion of a p-type device photocathode formed on the binary compound substrate material, the active device absorber layer comprising at least a quaternary or greater material structure configured to be lattice matched with the substrate material to reduce strain to allow charge carriers to go further in the active device absorber layer implemented in a photocathode of a nightvision system; and
forming a selective barrier layer on the active device absorber layer that is configured to act as a barrier for thermally generated electrons while freely passing energetic photogenerated electrons, wherein a thickness of the selective barrier layer is less than or equal to 10 nm.
15. The method of claim 14 , wherein the substrate material is GaAs and the active device absorber layer is InGaAsNSb or wherein the substrate material is InP and the active device absorber layer is InGaAsP.
16. The method of claim 15 further comprising forming an InGaP etch stop layer to prevent surface damage on the active device absorber layer when the substrate material is GaAs or forming an AlInAsP etch stop layer when the substrate layer is InP.
17. The method of claim 14 , further comprising doping the active device absorber layer formed on the binary compound substrate material exponentially by p-type impurities with levels of doping increasing away from an interface between the active device absorber layer and the binary compound substrate material, and wherein the p-type impurities comprise Be when the substrate material is GaAs or the p-type impurities comprise Zn when the substrate material is InP.
18. The method of claim 14 , wherein the step of forming the selective barrier layer comprises forming a fully strained GaAs layer when the substrate material is GaAs or a fully strained InP layer when the substrate material is InP, between an etch stop layer and the active device absorber layer.
19. The method of claim 18 , further comprising: removing the substrate material and the etch stop layer; and forming a Cs—O layer on the fully strained GaAs layer or the fully strained InP layer for activation.
20. The method of claim 14 , further comprising forming a window layer on the active device absorber layer.