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  • There are currently several known mechanisms for generating

    2018-10-24

    There are currently several known mechanisms for generating terahertz radiation, based on impurity-assisted transitions of charge carriers in semiconductors and semiconductor nanostructures. For example, terahertz apoptosis was observed during optical transitions of nonequilibrium charge carriers involving impurity resonance states under impurity breakdown in electric field in mechanically strained p-Ge [5] and in GaAs/GaAsN:Be microstructures with built-in stresses [6]. Additionally, terahertz radiation was observed from bulk silicon doped with various impurities under intraband optical excitation of charge carriers [7]. Terahertz radiation under interband photoexcitation was observed in doped bulk semiconductors such as GaN [8], GaAs and Ge [9]. There are few studies examining terahertz radiation from nanostructures with doped quantum wells (QWs). For example, terahertz radiation in longitudinal electric fields was observed in GaAs/AlGaAs quantum wells doped with donor [10] and acceptor [11] impurities. Terahertz radiation from nanostructures with doped QWs under interband optical pumping was first described in Ref. [11]. This type of pumping entails the generation of electron–hole pairs that are subsequently trapped in the QW. At low crystal lattice temperatures, donor impurities in the QWs are neutral. Electrons from donor ground states can recombine with nonequilibrium holes, which is usually accompanied by the emission of near-infrared photons. The impurity ground states depopulated as a result of this process can be filled with nonequilibrium electrons from the first subband of size quantization. This can occur with an emission of photons of the terahertz range. This study continues our previous studies on the subject [11] and is dedicated to examining radiation of the terahertz and near-IR ranges in nanostructures with donor-doped QWs of different widths.
    Samples and experimental procedure Optical studies were carried out for three samples. Two of them were grown by molecular-beam epitaxy on a semi-insulating gallium arsenide substrate and contained doped GaAs/AlGaAs QWs of different widths. The first sample contained 226 periods of GaAs QW 16.1nm in width, separated by 4.8-nm-thick Al0.15Ga0.85As barriers. The second sample contained 50 periods of GaAs QWs 30nm in width, separated by 7-nm-thick Al0.30Ga0.70As barriers. Structures with narrow and wide QWs had GaAs cap layers 60 and 20nm thick, respectively. The QWs in both structures were doped with silicon (acting as a donor) with a surface concentration =3·1010cm–2. A semi-insulating GaAs substrate, similar to those on which the nanostructures with doped QWs were grown, was used as the third reference sample. During optical measurements, the samples were mounted into a Janis PTCM-4-7 closed-cycle optical cryostat that allowed maintaining the sample\'s temperature in the range from 4 to 320K. The optical excitation of nonequilibrium charge carriers in the structures was carried out through a fused-quartz window by a continuous wave radiation of a solid-state diode-pumped laser (with the wavelength of λ=532nm and the average output power of P=8mW). The photoluminescence (PL) spectra in the terahertz spectral range were studied using a Bruker Vertex 80v vacuum Fourier transform infrared spectrometer operating in a step-scan mode. The output window of the optical cryostat was made of polymethylpentene, the entrance window of the spectrometer was made from polyethylene. These materials have a high degree of transparency in the terahertz spectral range. The PL radiation of the sample was collected by an off-axis parabolic mirror of the Fourier spectrometer through a black polyethylene filter that prevented the penetration of scattered pumping radiation into the measurement section of the experimental setup. A liquid helium cooled silicon bolometer, which had a vacuum contact with the spectrometer, was used as a detector of terahertz radiation. The signal of the bolometer photoresponse was measured by an SR830 lock-in amplifier which was synchronized with the pump laser. Laser radiation was modulated by a chopper at a frequency of 87Hz with a duty cycle of 50%.