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Description
InGaN/GaN multiple quantum wells (MQW) heterostructures exhibit a truly unique set of scintillation properties, both highly advantageous and detrimental. Scintillation efficiency is high, emission is nicely spectrally matched to common photomultipliers and SiPMs, and, most importantly, the emission is ultra-fast with decay time around 1 ns. On the other hand, the maximum thickness of the structure which has ever been achieved is only slightly above 2 micrometers, which is a fundamental limiting factor prohibiting application in many fields. Among considered applications, the time-of-flight mass spectrometry is the most notable.
Two samples have been selected for the presentation, MQW10s consisting of 10 alternating InGaN/GaN layers, MQW100 consisting of 100 such layers. Thickness of MQW region of samples MQW10s and MQW100 is 440 nm and 2600 nm, respectively. Samples have been characterized by cathodoluminescence, photoluminescence, scintillation decay measurement, and amplitude spectrometry. Amplitude spectrometry results will be discussed in greater details, other results only briefly.
Light yield of MQW100 is estimated to be 8 610 photons/MeV under alpha-particle excitation, and approximately 14 000 photons/MeV under X/gamma excitation. Value for MQW10s under X/gamma excitation is 35 000 photons/MeV. For MQW10s under alpha-particle excitation, only extremely rough estimation could be made, 13-21 000 photons/MeV. Interestingly, gamma/alpha ratios are unusually close to unity for a scintillator, probably due to highly efficient recombination of electrons and holes in MQWs. Ultra-fast emission and relatively high light yield make InGaN/GaN heterostructure a scintillator of one of the highest initial-photon time density (if not the highest), which is a crucial parameter determining timing resolution.