- #1
Hop-AC8NS
- 24
- 18
APD background to this discussion
There is a type of photon-activated device that goes into avalanche conduction by means of impact ionization, multiplying the conduction effect of a photon creating an electron-hole pair in a semiconductor junction. This type of device is called an Avalanche Photo-Diode or APD. It has much better quantum efficiency (approaching 90%) than a photo-multiplier tube or even a micro-channel electron multiplier and is just as sensitive to visible light.
Gallium Arsenide electrcal properties
If a device is made from a gallium arsenide crystal, a semiconductor junction is not necessary. GaAs has an unusually large electron mobility. It behaves as a light-sensitive photo-resistor at low bias voltages, decreasing in resistance with increasing light intensity. When biased at around four or five kilovolts per centimeter it becomes a photon-triggered switch.
The old GaAs PCSS
Sandia National Laboratories (SNL) in Albuquerque NM tried to use that effect to produce a high-speed, high-voltage, optically-triggered solid state switch. It had the nice property of remaining a conductor, much like a thyratron, after being triggered. The scientists at SNL call this "lock on" operation. Conduction persists until the current falls below a certain threshold that commutates the switch back to the non-conducting state.
SNL spent about a decade trying to solve a single problem: self-destruction from lightning-like discharge streamers that cause intense localized heating. SNL finally solved this problem by using cylindrical lenses to illuminate parallel-conduction trigger paths. The discharge is still "lightning-like" but now confined to narrow conduction channels. Back in the late 1990s this was deemed too expensive for their Z-pinch experiments, so they abandoned further work on developing GaAs Photo-Conductive Semiconductor Switches (GaAs PCSS).
The new and improved GaAs PCSS
Fast forward to the early 21st century. For reasons that are not really important, the Defense Threat Reduction Agency (DTRA) wanted my employer to try using oxygen ion implantation to create parallel, non-triggerable, "dead" zones on a GaAs PCSS. This would do the same thing that SNL's cylindrical lenses did but without any lens. Since I was already operating and maintaining a small particle accelerator, and implanting oxygen ions for isolation of GaAs integrated circuits (in lieu of etching for isolation) for a government customer, they asked if we could make a GaAs PCSS that would not self-destruct. It took awhile, but we succeeded. Our efforts received an R&D 100 award in 2015, but AFAIK the device has never been put into production.
Speculation and Experiments
It is not obvious WHY our ion-implanted GaAs PCSS works. The HOW to make one is simple: Prototypes were cut from a Chinese-manufactured 100mm diameter semi-insulating (un-doped) GaAs wafer. Each device was quite thick, optically, and about 10mm wide by 200mm long. Multi-layers of ohmic contacts were vacuum deposited on the flat surface at both ends of each prototype, leaving somewhat less than 190mm of conduction path between the two contacts. Then we did multiple-energy oxygen ion implants through a laser-slotted stainless steel mask, followed by annealing.
I designed, and had manufactured, a low-impedance sample holder that used gold-plated berrylium finger-contacts. These are normally used to seal doors in Faraday cages and other RF sensitive applications, so they are readily available. This design allowed kilo-ampere currents with picosecond rise times after optical triggering. The sample holder was designed to allow physical "floating" of the device, for thermal expansion of the PCSS after it was triggered, and for covering its surface with insulating Fluorinert liquid to prevent flash-over surface conduction in air.
More speculation without experimental tests
I believe that our PCSS conducted mostly in a few molecular layers under the surface after it was triggered. It responds to infrared as well as visible wavelengths by promoting an electron from the valence band to the conduction band when it intercepts a photon. Then the large electrical field, in conjunction with the large electron mobility, creates very rapid impact ionization leading to the "lock on" effect. The lightning-like discharge path is random and exhibits narrow filaments of conduction without ion implantation to guide the discharge. Even with narrow ion-implanted non-conduction channels, the current bounces back and forth between narrow conduction channels in an attempt to behave in a lightning-like manner.
Could this "lock on" effect be used as a quantum photon detector?
It has been decades since I was able to "play" with these "new and improved" GaAs PCSS devices, but the thought occurs to me that the "lock on" effect might be useful as a quantum photon detector if only a very thin layer of material is biased and used for photon detection.
I wonder if an electrically conductive layer could be interposed below a thin film a few micrometers thick of GaAs, and a transparent electrically conductive layer placed on top of the thin film. A few volts of bias would be applied between the two conductive layers, enough to cause avalanche impact ionization when triggered by a photon event but not otherwise causing any "dark" current because there is no semiconductor junction involved. If the entire wafer is patterned with GaAs "mesas" this could perhaps make a large-area photosensitive quantum detector.
Pushing the frontier of what is possible...
There are alternatives to using GaAs to make a PCSS, but they require a large photon flux and do not exhibit "lock on" characteristics. It was the goal of SNL to build a switch that could be triggered on, and stay on, with very much lower levels of photon flux. They succeeded, but decided their solution was impractical for their needs at the time. I am wondering what else has been overlooked or ignored since then because it either "wasn't invented here" or doesn't fit the existing paradigm? Does anyone here in PhysicsForums know how to make a thin-film GaAs device sandwiched between two conductors, one of which is transparent to visible light? A simple prototype for evaluation is desirable, but that is out of my wheelhouse.
There is a type of photon-activated device that goes into avalanche conduction by means of impact ionization, multiplying the conduction effect of a photon creating an electron-hole pair in a semiconductor junction. This type of device is called an Avalanche Photo-Diode or APD. It has much better quantum efficiency (approaching 90%) than a photo-multiplier tube or even a micro-channel electron multiplier and is just as sensitive to visible light.
Gallium Arsenide electrcal properties
If a device is made from a gallium arsenide crystal, a semiconductor junction is not necessary. GaAs has an unusually large electron mobility. It behaves as a light-sensitive photo-resistor at low bias voltages, decreasing in resistance with increasing light intensity. When biased at around four or five kilovolts per centimeter it becomes a photon-triggered switch.
The old GaAs PCSS
Sandia National Laboratories (SNL) in Albuquerque NM tried to use that effect to produce a high-speed, high-voltage, optically-triggered solid state switch. It had the nice property of remaining a conductor, much like a thyratron, after being triggered. The scientists at SNL call this "lock on" operation. Conduction persists until the current falls below a certain threshold that commutates the switch back to the non-conducting state.
SNL spent about a decade trying to solve a single problem: self-destruction from lightning-like discharge streamers that cause intense localized heating. SNL finally solved this problem by using cylindrical lenses to illuminate parallel-conduction trigger paths. The discharge is still "lightning-like" but now confined to narrow conduction channels. Back in the late 1990s this was deemed too expensive for their Z-pinch experiments, so they abandoned further work on developing GaAs Photo-Conductive Semiconductor Switches (GaAs PCSS).
The new and improved GaAs PCSS
Fast forward to the early 21st century. For reasons that are not really important, the Defense Threat Reduction Agency (DTRA) wanted my employer to try using oxygen ion implantation to create parallel, non-triggerable, "dead" zones on a GaAs PCSS. This would do the same thing that SNL's cylindrical lenses did but without any lens. Since I was already operating and maintaining a small particle accelerator, and implanting oxygen ions for isolation of GaAs integrated circuits (in lieu of etching for isolation) for a government customer, they asked if we could make a GaAs PCSS that would not self-destruct. It took awhile, but we succeeded. Our efforts received an R&D 100 award in 2015, but AFAIK the device has never been put into production.
Speculation and Experiments
It is not obvious WHY our ion-implanted GaAs PCSS works. The HOW to make one is simple: Prototypes were cut from a Chinese-manufactured 100mm diameter semi-insulating (un-doped) GaAs wafer. Each device was quite thick, optically, and about 10mm wide by 200mm long. Multi-layers of ohmic contacts were vacuum deposited on the flat surface at both ends of each prototype, leaving somewhat less than 190mm of conduction path between the two contacts. Then we did multiple-energy oxygen ion implants through a laser-slotted stainless steel mask, followed by annealing.
I designed, and had manufactured, a low-impedance sample holder that used gold-plated berrylium finger-contacts. These are normally used to seal doors in Faraday cages and other RF sensitive applications, so they are readily available. This design allowed kilo-ampere currents with picosecond rise times after optical triggering. The sample holder was designed to allow physical "floating" of the device, for thermal expansion of the PCSS after it was triggered, and for covering its surface with insulating Fluorinert liquid to prevent flash-over surface conduction in air.
More speculation without experimental tests
I believe that our PCSS conducted mostly in a few molecular layers under the surface after it was triggered. It responds to infrared as well as visible wavelengths by promoting an electron from the valence band to the conduction band when it intercepts a photon. Then the large electrical field, in conjunction with the large electron mobility, creates very rapid impact ionization leading to the "lock on" effect. The lightning-like discharge path is random and exhibits narrow filaments of conduction without ion implantation to guide the discharge. Even with narrow ion-implanted non-conduction channels, the current bounces back and forth between narrow conduction channels in an attempt to behave in a lightning-like manner.
Could this "lock on" effect be used as a quantum photon detector?
It has been decades since I was able to "play" with these "new and improved" GaAs PCSS devices, but the thought occurs to me that the "lock on" effect might be useful as a quantum photon detector if only a very thin layer of material is biased and used for photon detection.
I wonder if an electrically conductive layer could be interposed below a thin film a few micrometers thick of GaAs, and a transparent electrically conductive layer placed on top of the thin film. A few volts of bias would be applied between the two conductive layers, enough to cause avalanche impact ionization when triggered by a photon event but not otherwise causing any "dark" current because there is no semiconductor junction involved. If the entire wafer is patterned with GaAs "mesas" this could perhaps make a large-area photosensitive quantum detector.
Pushing the frontier of what is possible...
There are alternatives to using GaAs to make a PCSS, but they require a large photon flux and do not exhibit "lock on" characteristics. It was the goal of SNL to build a switch that could be triggered on, and stay on, with very much lower levels of photon flux. They succeeded, but decided their solution was impractical for their needs at the time. I am wondering what else has been overlooked or ignored since then because it either "wasn't invented here" or doesn't fit the existing paradigm? Does anyone here in PhysicsForums know how to make a thin-film GaAs device sandwiched between two conductors, one of which is transparent to visible light? A simple prototype for evaluation is desirable, but that is out of my wheelhouse.