Dual band photodiode element and method of making the same
Mercury cadmium telluride (MCT) dual band photodiode elements are described that include an n-type barrier region interposed between first and second p-type regions. The first p-type region is arranged to absorb different IR wavelengths to the second p-type region in order that the photodiode element can sense two IR bands. A portion of the second p-type region is type converted using ion-beam milling to produce a n-type region that interfaces with the second p-type region and the n-type barrier region.
1. A method of manufacturing a photodiode element responsive to dual band radiation, the method comprising:
forming a first p-type region;
forming a first n-type region that includes an n-type barrier layer on the first p-type region;
forming a second p-type region on the first n-type region;
using ion beam milling to type convert a portion of the second p-type region to provide a second n-type region that is in electrical and physical contact with both the second p-type region and the first n-type region; and
providing a first metallic contact on the second p-type region and a second metallic contact on the second n-type region, the second metallic contact being provided over a recess formed by the ion beam milling process.
2. A method according to claim 1 , wherein the second p-type region is formed from mercury-cadmium-telluride.
3. A method according to claim 1 , comprising:
forming a passivation layer over the photodiode element and a window in the passivation layer through which at least a portion of the second p-type region is exposed; and
ion beam milling through the window to convert the exposed portion of the second p-type region to the second n-type region.
4. A method according claim 3 , comprising:
depositing a metal through the window to form a contact.
5. A method according to claim 4 , comprising:
forming a mask over the photodiode element that leaves the window exposed;
ion beam milling through the mask to convert the exposed portion of the second p-type region to the second n-type region; and
depositing a metal through the mask to form a contact with the n-type region.
6. A method according to claim 3 , comprising:
before forming the passivation layer, providing a second mask over the photodiode element, forming the passivation layer over the photodiode element, and removing the second mask to provide the window in the passivation layer to expose the portion of the second p-type region.
7. A method according to claim 1 , comprising:
etching through the second p-type region, first n-type region and first p-type region to form a mesa photodiode element.
8. A method according to claim 1 , comprising:
growing a p-type common layer on a substrate and growing the first p-type region on the p-type common layer.
9. A method according to claim 1 , wherein the first p-type region is configured to absorb a shorter waveband and the second p-type region is configured to absorb a longer waveband.
10. A method according to claim 1 , wherein a first portion of the first n-type region is configured to absorb a shorter waveband and a second portion of the first n-type region is configured to absorber a longer waveband.
11. A method according to claim 1 , wherein the second n-type region is configured to absorb the longer waveband.