2. Articole
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Item QUANTUM NANOSTRUCTURES FOR TERAHERTZ DEVICES AND APPLICATIONS(2019) Sirkeli, Vadimerahertz (THz) waves refer to the electromagnetic radiation in the frequency range from 0.1 to 10 THz, which corresponds to the wavelengths from 3 mm to 30 μm, respectively. This spectral region, called also as “T-gap”, is important for many practical applications, including THz imaging, chemical and biological sensing, high-speed telecommunication, security and medical applications. Here we report the results of a numerical study of quantum transport in ZnO-based resonant-tunneling diodes (RTDs) and quantum cascade lasers (QCLs) with different design schemes. We found that by varying and optimizing constituent layer widths and doping level these quantum structures, high power performance of THz RTDs and QCLs can be achieved at room temperature. Moreover, it was established also that the ZnO-based terahertz sources can cover the spectral region of 5-12 THz, which is very important for THz imaging and detection of explosive materials, and which could be not covered by conventional GaAs-based terahertz devices.Item HIGH PERFORMANCE ZnSe-BASED ULTRAVIOLET PHOTODETECTORS WITH Cr/Au, Ni/Au AND HYBRID Ag-NANOWIRE CONTACTS(2024) Sirkeli, Vadim; Nedeoglo, Natalia; Nedeoglo, Dmitrii; Yilmazoglu, Oktay; Hajo, Ahid; Preu, Sascha; Küppers, Franko; Hartnagel, HansItem TERAHERTZ RESONANT TUNNELING DIODES BASED ON GaN/AlGaN STRUCTURES(CEP USM, 2022-11-10) Sirkeli, VadimItem RECENT PROGRESS IN WIDE BANDGAP SEMICONDUCTORS-BASED DEVICES FOR TERAHERTZ APPLICATIONS(CEP USM, 2020) Sirkeli, VadimItem RECENT PROGRESS IN GaN-BASED DEVICES FOR TERAHERTZ TECHNOLOGY(Springer Nature, 2020) Sirkeli, Vadim; Tiginyanu, Ion; Hartnagel, HansThis paper reviews the crystal growth, basic properties, and principle of operation of III-nitride based terahertz devices. We provide a brief history and current status of crystal growth of polar and non-polar GaN-based heterostructures and its properties. The role of spontaneous and piezoelectric polarization in polar III-nitride structures and its impact on performance of terahertz devices is discussed in detail. We show that GaN-based semiconductor compounds are promising materials for fabrication terahertz sources operating up to room temperature due to their unique properties such as large bandgap and conduction band offset (CBO) energy, high LO-phonon energy, and high resistant to the high breakdown electric field. Moreover, it was established that the GaN-based terahertz sources can cover the spectral region of 5–12 THz, which is very important for THz imaging and detection of explosive materials, and which could be not covered by conventional GaAs-based terahertz devices. In terms of the reported significant progress in growth of non-polar m-plane GaN-based heterostructures and devices with low density defects, it is open a wide perspective towards design and fabrication of non-polar m-plane GaN-based high power terahertz sources with capabilities of operation at room temperature.Item ZnO-BASED QUANTUM STRUCTURES FOR TERAHERTZ SOURCES(Springer Nature, 2020) Sirkeli, Vadim; Hartnagel, Hans; Yilmazoglu, O.; Preu, S.In this paper we report on the numerical study of the terahertz devices based on metal oxide semiconductors and its application in biology and medicine. We also report on the recent progress of the theoretical and experimental studies of ZnO-based THz quantum cascade lasers (QCLs) and resonant tunneling diodes (RTDs). We show that ZnO-based semiconductor compounds are promising materials for fabrication terahertz sources operating up to room temperature due to their unique properties such as large bandgap and conduction band offset (CBO) energy, high LO-phonon energy, and high resistant to the high breakdown electric field. Moreover, it was established that the ZnO-based terahertz sources can cover the spectral region of 5–12 THz, which is very important for THz imaging and detection of explosive materials, and which could be not covered by conventional GaAs-based terahertz devices. In terms of the reported significant progress in growth of non-polar m-plane ZnO-based heterostructures and devices with low density defects, it is open a wide perspective towards design and fabrication of non-polar m-plane ZnO-based high power terahertz sources with capabilities of operation at roomItem THE USE OF METAL OXIDE SEMICONDUCTORS FOR THZ SPECTROSCOPY OF BIOLOGICAL APPLICATIONS(Springer Nature, 2019) Hartnagel, Hans Ludwig; Sirkeli, VadimTerahertz (THz) waves refer to the electromagnetic radiation in the frequency range from 0.1 to 10 THz, which corresponds to the wavelengths from 3 mm to 30 µm, respectively. This spectral region, called also as “T-gap”, is important for many practical applications, including THz imaging, chemical and biological sensing, high-speed telecommunication, security and medical applications. THz waves have low photon energies (~4.1 meV for 1 THz), which is about 1 million times weaker than the energy of X-ray photons. They do neither ignite any explosive materials at typical power levels nor cause any harmful ionization in biological tissues. The terahertz radiation is strongly attenuated by water and is very sensitive to water content. Unique THz absorption spectra caused by intermolecular vibrations in this spectral region have been found in different biological materials and tissues. Thus, Terahertz spectroscopy provides a powerful tool for characterization of a great many bio molecules and tissues. All these applications require relatively high power terahertz sources with milliwatt-level output power, which could operate at room temperature. Despite great progress, made in the last few years of design, fabrication and demonstration, THz devices based on GaAs/AlGaAs materials, there are some limits of bandgap engineering due to the relatively low (0.72 eV for GaAs/AlAs) conduction band offset, and most terahertz sources with one milliwatt-power like quantum cascade lasers (QCLs) require cryogenic cooling down to less than 200 K. To overcome the issue the new material systems such as metal oxide materials are considered as promising for room-temperature THz sources. The interest in terahertz imaging and spectroscopy of biologically related applications is increasing more and more within the last few years. This paper provides a review and current status of using metal oxide materials for THz spectroscopy, and recent advances in terahertz spectroscopy techniques in biological and medical applications.Item ZnO-BASED TERAHERTZ QUANTUM CASCADE LASERS(Elsevier, 2019) Sirkeli, Vadim; Hartnagel, HansHigh-power terahertz sources operating at room-temperature are promising for many applications such as explosive materials detection, non-invasive medical imaging, and high speed telecommunication. Here we report the results of a simulation study, which shows the significantly improved performance of room-temperature terahertz quantum cascade lasers (THz QCLs) based on a ZnMgO/ZnO material system employing a 2-well design scheme with variable barrier heights and a delta-doped injector well. We found that by varying and optimizing constituent layer widths and doping level of the injector well, high power performance of THz QCLs can be achieved at room temperature: optical gain and radiation frequency is varied from 108 cm−1 @ 2.18 THz to 300 cm−1 @ 4.96 THz. These results show that among II–VI compounds the ZnMgO/ZnO material system is optimally suited for high-performance room-temperature THz QCLs.Item PURIFICATION OF ZNSE CRYSTALS FROM ELECTRICALLY ACTIVE BACKGROUND IMPURITIES BY YTTERBIUM DOPING(John Wiley & Sons, 2014) Radevici, Ivan; Sushkevich, Konstantin; Sirkeli, Vadim; Nedeoglo, Dmitrii; Nedeoglo, Natalia; Huhtinen, Hannu; Paturi, PetriinaHall coefficient, electrical conductivity, and electron mobility are investigated for n-ZnSe:Yb single crystals with high concentration of electrically active background impuritiesItem RECENT ADVANCES IN TERAHERTZ TECHNOLOGY FOR SECURITY AND BIOMEDICAL APPLICATIONS(Universitatea de Stat din Tiraspol, 2021) Sirkeli, VadimTerahertz waves have low photon energies (~ 4.1 meV for 1 THz), which is about 1 million times weaker than the energy of X-ray photons. They do not cause any harmful ionization in biological tissues. The terahertz radiation is strongly attenuated by water and is very sensitive to water content. This paper provides current status and recent advances in terahertz technology for security and medical applications. In particular, we report on our designs of THz quantum cascade lasers to identify cancerous tissues and other medical issues.
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