Light-emitting devices based on semiconductor quantum dots have been shown to be suitable for use in environments that include high levels of radiation that causes displacement damage in semiconductors - in particular, energetic protons. Conventional light-emitting diodes and other conventional optoelectronic devices become degraded rapidly by such radiation, giving rise to a need for radiation-hard devices. A preliminary confirmation of the feasibility of using semiconductor quantum dots to fill this need has been provided by experimental observations of radiation hardness in InxGa1-xAs/GaAs quantum dots.
For the experiments, specimens containing InxGa1-xAs/GaAs quantum dots [lenslike islands (≈5 nm thick and ≈25 nm in diameter) of InxGa1-xAs surrounded by GaAs] were fabricated by metal-organic chemical vapor deposition of In0.6Ga0.4As and GaAs on GaAs substrates. For comparison, specimens containing quantum wells (as distinguished from quantum dots) were also fabricated by stopping the growth of In0.6Ga0.4As before the onset of the Stranski-Krastanow transformation [in which quantum dots form spontaneously in a second semiconductor (in this case, In0.6Ga0.4As) deposited on a lattice-mismatched first semiconductor (in this case, GaAs) once the second semiconductor reaches a critical thickness, which is typically a few molecular layers].
In the experiments, the specimens were irradiated with protons at a kinetic energy of 1.5 MeV from a Van de Graaff generator. Next, the light-emitting properties of specimens that had been exposed to a range of proton doses were evaluated in terms of photoluminescence emitted by the specimens at various temperatures. The photoluminescence was excited by light at a wavelength of 514 nm from an argon-ion laser and measured by use of a cooled germanium detector and a lock-in detection technique.
The figure shows the measured integrated normalized photoluminescence intensities from the quantum wells and quantum dots as functions of the proton dose, the normalization being with respect to the zero-dose values. These plots suggest that quantum dots can tolerate about 50 to 100 times as much radiation as quantum wells can. The increase in radiation hardness of quantum dots over quantum wells is all the more significant in that quantum-well optoelectronic devices (e.g., light-emitting diodes) based on quantum wells have already been demonstrated to be an order of magnitude more radiation-hard than are the corresponding conventional optoelectronic devices (e.g., light-emitting diodes based on p/n junctions).
This work was done by Rosa Leon of Caltech for NASA's Jet Propulsion Laboratory.
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Semiconductor Quantum Dots as Radiation-Hard Light Emitters
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