Abstract:Avalanche photodiode (APD) has been intensively investigated as a promising candidate to replace the bulky and fragile photomultiplier tubes (PMT) for weak light detection. However, intrinsic limits in semiconductors make the former still inferior to the latter on device performance up to now. In conventional APDs, a large portion of carrier energy drawn from the electric field is thermalized, and the multiplication efficiencies of electron and hole are low and close. In order to achieve high gain, the device has to work under breakdown bias, wherein carrier multiplication proceeds bi-directionally to form a positive feedback multiplication circle. In this case, APDs should work under Geiger mode as a compromise between sustainable detection and high gain. On the other hand, PMT can achieve stable high gain under constant bias (linear mode). Here, we demonstrate an APD works like a PMT, which means it can work under constant bias and holds high gain without breakdown simultaneously. The device is based on a GaN/AlN periodically-stacked-structure (PSS). For the PSS holds the intrinsic features that there are deep Γ valleys and larger band offset in conduction band, electron encountered much less scatterings during transport in PSS APD. Electron holds much higher efficiency than hole to draw energy from the electric field, and avalanche happens uni-directionally with high efficiency. Extremely high ionization coefficient (3.96×10 5 /cm) of electron and large ionization coefficient ratio (over 100) between electron and hole is calculated in the PSS APD by Monte-Carlo simulations and a recorded high gain (10 4 ) tested under constant bias without breakdown is obtained in a prototype device, wherein the stable gain can be determined by the periodicity of the GaN/AlN PSS and no quenching circuits are needed for sustainable detection. This work not only brings a new light into avalanche multiplication mechanism, but also paves a technological path to realize highly sensitive APD working under constant bias like PMT.Keywords: Avalanche photodiode; GaN; Ionization; Periodically stacked structure; Transport Introduction:
Self-assembled Al y Ga1− y N quantum dots (QDs), with y = 0 and 0.1, have been grown by molecular beam epitaxy on Al0.5Ga0.5N(0001) oriented layers using sapphire substrates. The QD formation has been followed in situ by reflection high energy electron diffraction (RHEED). A two- to three-dimensional (2D–3D) transition of the layer morphology is observed, characterized by a change of the RHEED pattern from streaky lines to Bragg spots. High QD densities, from 1010 up to near 1012 cm−2, have been obtained. By decreasing the GaN QD size and incorporating Al inside the QDs, a strong variation in the photoluminescence (PL) emission has been observed, enabling to cover a large spectral range from near UV (3 eV) to UV-B (3.95 eV). By combining temperature-dependent and time-resolved PL measurements, the internal quantum efficiency of the QDs has been determined at both low and high temperatures as a function of the PL energy.
We report the growth by molecular beam epitaxy of highly resistive GdN, using intentional doping with magnesium.
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