Substrate stack epitaxies for photocathodes for extended wavelengths

A photocathode epitaxial structure comprising:

• an improved substrate stack, the improved substrate stack comprising:
• a GaAs substrate;
• and
• one or more additional layers formed on the GaAs substrate, the one or more additional layers configured to provide an improved substrate stack surface with predetermined characteristics for forming a semiconductor device on the improved substrate stack surface;
• and
• an InGaAs p-type photocathode formed on the improved substrate stack surface, the InGaAs p-type photocathode having a predetermined percentage of In,
• wherein the improved substrate comprises a plurality of step graded InGaAs layers which gradually increase an amount of Indium doping from layer to layer from a first layer more proximate the GaAs substrate to an overshoot layer less proximate the GaAs substrate, wherein the overshoot layer has a first amount of Indium doping that is greater than second amount of doping in the InGaAs p-type photocathode to reduce strain in layers from the GaAs substrate to the InGaAs p-type photocathode.

A method of forming a photocathode, the method comprising:

• forming an improved substrate stack by forming:
• a GaAs substrate;
• and
• one or more additional layers on the GaAs substrate, the one or more additional layers configured to provide an improved substrate stack surface with predetermined characteristics for forming a semiconductor device on the improved substrate stack surface;
• and
• forming an InGaAs p-type photocathode on the improved substrate stack surface, the InGaAs p-type photocathode having a predetermined percentage of In,
• wherein forming the one or more additional layers on the GaAs substrate comprises forming an overshoot layer having a first amount of Indium doping that is greater than second amount of doping in the InGaAs p-type photocathode to reduce strain.

A method of forming a photocathode, the method comprising:

• forming an improved substrate stack by forming:
• a GaAs substrate;
• and
• one or more additional layers on the GaAs substrate, the one or more additional layers configured to provide an improved substrate stack surface with predetermined characteristics for forming a semiconductor device on the improved substrate stack surface;
• and
• forming an InGaAs p-type photocathode on the improved substrate stack surface, the InGaAs p-type photocathode having a predetermined percentage of In,
• wherein forming the one or more additional layers on the GaAs substrate comprises forming a plurality of step graded InGaAs layers which gradually increase an amount of Indium doping from layer to layer from a first layer more proximate the GaAs substrate to an overshoot layer less proximate the GaAs substrate, wherein the overshoot layer has a first amount of Indium doping that is greater than second amount of doping in the InGaAs p-type photocathode to reduce strain in layers from the GaAs substrate to the InGaAs p-type photocathode.

A photocathode epitaxial structure comprising:

• an improved substrate stack, the improved substrate stack comprising:
• a GaAs substrate;
• a plurality of step graded InGaAs layers which gradually increase an amount of Indium doping from layer to layer from a first layer more proximate the GaAs substrate to a final layer less proximate the GaAs substrate;
• and
• an InGaAs p-type photocathode formed on the improved substrate stack, the InGaAs p-type photocathode having a predetermined percentage of In;
• and
• wherein the final layer is an overshoot layer having a first amount of Indium doping that is greater than second amount of doping in the InGaAs p-type photocathode, and wherein the overshoot layer is thick as compared to other layers between the overshoot layer and the GaAs substrate.