|
Ultrafast electron spectroscopy – the use of sub-ns pulses of electrons to excite materials of interest – offers many opportunities in the study of novel ultra-violet emitters and detectors. Due to the variability of electron energy one may go from tens of electronvolts to tens of Kiloelectronvolts; from probing the dynamics of conduction-band states to penetrating bulk samples in the interest of interacting with unoccupied states and measuring lattice scattering, tasks that traditional photon-based ultrafast spectroscopy is simply unable to perform. Furthermore, the added capability of time-correlated measurement allows for these properties to be studied in an unprecedented way. Presented is a photo-excited electron gun system capable of sub-ns pulses used to probe phosphors for betaphotovoltaics and the carrier dynamics of Nitride heterostructures grown for the purpose of UV detection and emission.
The electron gun, itself, is a Kimball-built system capable of electron acceleration up to, and including, 30keV. Frequency-doubled light from a 250-kHz Coherent Ti:sapphire laser system is focussed on a LaB6 cathode, capable of either thermionic emission or photoemission. The electron gun allows for the exchange of photocathodes to test novel emitters in the interests of further advancing and refining the electron gun system’s capabilities. Emitted electrons are then focussed and diverted using magnetic optics and onto the sample. Light emission from the sample is collected and directed out of the vacuum chambre and into a PMT-enabled monochromator. The use of a PMT allows for single-photon counting and accurate time-correlated measurement.
The phosphors studied were LaPO4:Pr and ZnS, photon emitters to be paired with InGaN and InGaP, respectively, for the purposes of betaphotovoltaic power generation – a long-lived power source. The use of the 30keV-capable electron gun is ideal for these purposes as it is able to measure the phosphor’s beta response over a broad energy spectrum, covering the entire spectrum of tritium and most of that of 63Ni, our two candidates for beta sources. The combination of a UV source and detector bearing the same efficiency as the visible source would allow for betaphotovoltaic generators with greater tension for more varied applications.
Research has also been performed on InGaN/GaN structures for solar-blind UV detection and emission. Moving into the UV region has introduced challenges for which electron sources are perfectly suited; from the difficulty of p-type growth at these energy levels to the need to study carrier dynamics in novel materials.
None of this is to say that the generation of these bunches of electrons is a trivial matter: that electrons are charged introduces temporal and spatial broadening that is difficult to overcome, the collection of emitted signals introduces complications, and the measurement of pulses of electrons shorter than 1ns is difficult with the current state of detector technology, but these problems provide the opportunity for new solutions, as well.
|
