A weakly coupled semiconductor superlattice as a harmonic hypersonic-electrical transducer

We study experimentally and theoretically the effects of high-frequency strain pulse trains on the charge transport in a weakly coupled semiconductor superlattice. In a frequency range of the order of 100 GHz such excitation may be considered as single harmonic hypersonic excitation. While travelling along the axis of the SL, the hypersonic acoustic wavepacket affects the electron tunnelling, and thus governs the electrical current through the device. We reveal how the change of current depends on the parameters of the hypersonic excitation and on the bias applied to the superlattice. We have found that the changes in the transport properties of the superlattices caused by the acoustic excitation can be largely explained using the current-voltage relation of the unperturbed system. Our experimental measurements show multiple peaks in the dependence of the transferred charge on the repetition rate of the strain pulses in the train. We demonstrate that these resonances can be understood in terms of the spectrum of the applied acoustic perturbation after taking into account the multiple reflections in the metal film serving as a generator of hypersonic excitation. Our findings suggest an application of the semiconductor superlattice as a hypersonic-electrical transducer, which can be used in various microwave devices

30 μm thick GaAs X-ray p+-i-n+ photodiode grown by MBE

A GaAs p+-i-n+ photodiode detector with a 30 μm thick i layer and a 400 μm diameter was processed using standard wet chemical etching from material grown by molecular beam epitaxy. The detector was characterized for its electrical and photon counting X-ray spectroscopic performance at temperatures from 60°C to -20 °C. The leakage current of the detector decreased from 1.247 nA ± 0.005 nA (= 0.992 μA/cm2 ± 0.004 μA/cm2) at 60 °C to 16.0 pA ± 0.5 pA (= 12.8 nA/cm2 ± 0.4 nA/cm2) at -20 °C, at the maximum investigated applied reverse bias, -100 V (corresponding to an applied electric field of 33 kV/cm). An almost uniform effective carrier concentration of 7.1 × 1014 cm-3 ± 0.7 × 1014 cm-3 was found at distances between 1.7 μm and 14 μm below the p+-i junction, which limited the depletion width to 14 μm ± 1 μm, at the maximum applied reverse bias (-100 V). Despite butterfly defects having formed during the epitaxial growth, 55 Fe X-ray spectra were successfully obtained with the detector coupled to a custom-made charge-sensitive preamplifier; the best energy resolution (Full Width at Half Maximum at 5.9 keV) improved from 1.36 keV at 60 °C to 0.73 keV at -20 °C. Neither the leakage current nor the capacitance of the GaAs detector were found to be the limiting factors of the energy resolution of the spectroscopic system; noise analysis at 0 °C and -20 °C revealed that the dominant source of noise was the quadratic sum of the dielectric and incomplete charge collection noise