Why is the oscillator strength in superlattice detectors lower than in a p-n junction? What about QWIP detectors?
The correct answer and explanation is:
The oscillator strength in superlattice detectors is lower than in a p-n junction due to the unique structure and properties of the superlattice. A superlattice is made up of alternating layers of materials with different bandgaps, creating a periodic potential that affects the electronic states. In such structures, the electron transitions that are responsible for absorption and emission of light tend to be weaker compared to those in a p-n junction. This is because the periodic potential in the superlattice can result in the splitting of energy bands, leading to a reduced overlap between the electron and hole wavefunctions. As a result, the probability of electron transitions (which is directly related to oscillator strength) decreases.
On the other hand, in a p-n junction, the absorption process is typically more efficient due to the direct overlap of the wavefunctions of the electrons and holes. This leads to a higher oscillator strength, which results in stronger light absorption and emission.
For Quantum Well Infrared Photodetectors (QWIPs), the oscillator strength can also be lower compared to p-n junction detectors, but for different reasons. In QWIPs, the detector structure consists of quantum wells where the electron movement is confined in one direction, leading to discrete energy levels. The electron transitions in these quantum wells can be weaker because the confinement reduces the overlap between the electron and hole wavefunctions. Additionally, the transitions in QWIPs are typically optically forbidden, meaning that they are less probable, further reducing the oscillator strength. This makes QWIPs less efficient at absorbing light compared to p-n junction detectors, but they can still be effective due to their unique ability to detect infrared light.
In conclusion, the lower oscillator strength in superlattice detectors and QWIP detectors is due to the reduced overlap of electron and hole wavefunctions caused by their unique structural properties. These factors result in less efficient transitions compared to conventional p-n junction detectors.