High-efficiency, picosecond scintillation detectors are critically needed for many applications, particularly in the fields of medical imaging, high energy physics and nuclear security. Recently, we proposed a design and considerations for a semiconductor scintillator composed of InAs Quantum Dots (QDs) acting as luminescence centers in a GaAs stopping matrix/waveguide, integrated with a photodetector grown epitaxially on top of the waveguide (WG) [1]. This material has appealing potential photonic properties including high light yield (~240,000 photons/MeV) due to the narrow semiconductor bandgap, fast capture of electrons in QDs (2-5ps) and high radiation hardness (>104 Gy). The high refractive index of GaAs (n=3.4) ensures light emitted by the QDs is waveguided within the GaAs matrix, which can then be collected by an integrated InGaAs p-i-n photodiode (PD). A proof-of-concept integrated scintillation detector (Fig. 1) was grown using molecular beam epitaxy on a GaAs substrate. It has a 20 μm thick GaAs layer with embedded sheets of modulation p-type doped InAs QDs. A generic excitation source is shown interacting with the GaAs matrix, resulting in QD luminescence, which is then waveguided towards an InGaAs PD, grown on top of the WG and tuned to the QD luminescence band (~1150 nm). Employing QD structure engineering improves the internal efficiency of the QD luminescence to ~60% at room temperature and reduces overall waveguide attenuation to about 3-4 cm-1. A sample, as shown in Fig. 1, was used to evaluate the timing characteristics of 5.5 MeV alpha particle responses. The source had a radioactivity of 1 µCi, suspended in air approximately 1 cm above the QD waveguide. The scintillation response (Fig. 2) had a decay time of 0.3 ns, corresponding to QD recombination time, and a noise-limited time resolution of 54 ps (Fig. 3). With a 2D waveguide and an integrated PD covering only 16% of the WG width, the collected charge averaged 1.5x105 electrons (Fig. 4), corresponding to a collection efficiency of about 14%. This can be estimated as 34,000 photoelectrons per 1 MeV of incident energy, given about 1 MeV of alpha particle energy loss in air. This data confirms the unique photonic properties of this scintillation detector, which has the potential to be much faster than any currently used inorganic scintillator. Additionally, technologies for separating an epitaxial film from a substrate and bonding multiple waveguides using organic adhesives are tested and evaluated. [1] S. Oktyabrsky, et al. "Integrated Semiconductor Quantum Dot Scintillation Detector: Ultimate Limit for Speed and Light Yield," IEEE TNS, 63, 656 (2016). Figure 1
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Dec 1, 2023
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