Method of forming variable sensitivity transmission mode negative electron affinity photocathode

Claim 3

A method as set forth in claim 1 wherein said step of providing a large bandgap single crystal transparent window substrate having a wide bandgap semi-insulating window layer thereon is comprised of providing a single crystal seed substrate and epitaxially growing a conductor layer on said seed substrate followed by a step of epitaxially growing a high resistivity semi-insulating layer for providing a transparent conductor-insulator combination wherein the transparent conductor acts as a field plate and the transparent insulator acts as the dielectric material through which the electric field is applied and removing at least a portion of said seed substrate if it is not transparent to the wavelength of interest to define an active window area.

Claim 4

A method as set forth in claim 2 wherein said step of providing a transparent window substrate is comprised of providing a (100)-oriented GaAs single crystal seed substrate of about 15 mils thickness which is p-doped with Zinc to 5×10 18 carriers cm -3 and is polished on the growth surface with a 5H 2 SO 4 :1H 2 O 2 :1H 2 O etch to remove work damage, epitaxially growing a GaP conductor layer on said GaAs single crystal seed substrate using a HCl-Ga-PH 3 -H 2 vapor process, epitaxially growing an approximately 0.5 micron thich Oxygen or Chromium doped high resistivity (≧10 10 ohm-cm) GaP semi-insulating layer using the HCl-Ga-PH 3 -H 2 vapor process, and removing at least a portion of said GaAs single crystal seed substrate thereby exposing a portion of said GaP conductor layer, whereby to define an active window area, wherein said step of applying an antireflection coating is comprised of coating said exposed portion of said GaP conductor layer, wherein said step of epitaxially growing a photoemitter layer is comprised of growing a p-type Zinc-doped to 5×10 18 carriers cm 3 approximately one micron thick GaAs photoemitting layer onto said GaP semi-insulating layer using a HCl-Ga-AsH 3 -H 2 vapor process in which the GaP substrate and the GaAs photoemitter layer provide a broad ban response to about 0.93 microns with a short wavelength cutoff of about 0.56 microns, and wherein said step of activating said photoemitter layer is comprised of heating said GaAs photoemitter layer to approximately 610° C. in vacuum to clean its surface and activating said photoemitter layer with layers of Cesium and Oxygen to bring about a condition of negative electron affinity.

Claim 5

A method as set forth in claim 3 wherein said step of epitaxially growing a photoemitter layer is further comprised of incorporating Indium therein to form a lower bandgap GaInAs ternary emitting layer to extend the long wavelength response beyond 0.93 microns.

Claim 6

A method as set forth in claim 1 wherein said step of providing a large bandgap single crystal transparent window substrate is comprised of preparing a single crystal (111B) oriented p-type Zinc-doped GaP conductor seed crystal of about 15 mils thickness for epitaxial growth and epitaxially growing a high resistivity semi-insulating layer of GaAlAs to about one micron thickness by liquid phase epitaxy onto said GaP conductive seed crystal by using a sliding boat technique for providing a transparent conductor-insulator combination, and wherein the further step of epitaxially growing a photoemitter layer on said transparent window substrate is comprised of growing a p-type Zinc-doped GaAs photoemitter layer of about one micron thickness by liquid phase epitaxy onto said semi-insulating layer using a sliding boat technique.