Browsing by Author "Hartnagel, Hans"

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EFFECT OF p-NiO AND n-ZnSe INTERLAYERS ON THE EFFICIENCY OF p-GaN/n-ZnO LIGHT-EMITTING DIODE STRUCTURES
(IOP Publishing Ltd, 2015) Sirkeli, Vadim; Yilmazoglu, Oktay; Küppers, Franko; Hartnagel, Hans
We report on a numerical study of the characteristics of p-GaN/n-ZnO light-emitting diodes (LEDs) with p-NiO and n-ZnSe interlayers, and on LED design optimization which includes bandgap engineering, thickness and doping of constituent layers. The current-voltage dependences of investigated LEDs show a threshold voltage of 3.1 V, 5.4 V and 5.6 V for LED devices without and with the presence of p-NiO and n-ZnSe interlayers, respectively. It is found that p-NiO, n-ZnSe and n-ZnO interlayers act as an electron blocking layer, active media layer, and electron transport layer, respectively. It is established that the insertion of both p-NiO and n-ZnSe interlayers leads to the enhancement of charge carrier-confinement in the active region and to the significant increase of internal quantum efficiency (IQE) of the LED device up to 82%, which is comparable with IQE values in order to obtain better AlGaN- and InGaN-based LEDs. It is found that the efficiency of LED devices at 100 A cm−2 is equal to 0.024, 0.09 and 16.4% of external quantum efficiency (EQE), 1.3 × 10−4, 1.6 × 10−4, and 6.4 lm W−1 of PE, and 1.3 × 10−4, 2.9 × 10−4, and 12 cd A−1 of CE for p-GaN/n-ZnO, p-GaN/p-NiO/n-ZnO, and p-GaN/p-NiO/n-ZnSe/n-ZnO LED devices, respectively.
EFFECT OF p-NiO INTERLAYER ON INTERNAL QUANTUM EFFICIENCY OF p-GaN/n-ZnO LIGHT-EMITTING DEVICES
(American Scientific Publishers, 2015) Sirkeli, Vadim; Yilmazoglu, Oktay; Küppers, Franko; Hartnagel, Hans
We report on numerical investigations of p-GaN/n-ZnO light-emitting devices with p-NiO interlayer, and on LED design optimization which includes bandgap engineering, thickness and doping of constituent layers. The current–voltage dependences of investigated LEDs show a threshold voltage of 3.1 V and 5.4 V for the LED devices without and with presence of p-NiO interlayer, respectively. It is found that p-NiO layer act as electron blocking layer, that lead to the enhance of charge carriers confinement in active region, and to the increasing of internal quantum efficiency (IQE) of LED device up to 0.5%, that in four times higher in compare with that for original p-GaN/n-ZnO LED device.
ENHANCED RESPONSIVITY OF ZnSe-BASEDMETAL–SEMICONDUCTOR–METAL NEAR-ULTRAVIOLETPHOTODETECTOR VIA IMPACT IONIZATION
(Willey, 2018) Sirkeli, Vadim; Yilmazoglu, Oktay; Hajo, Ahid S.; Nedeoglo, Natalia; Nedeoglo, Dmitrii; Preu, Sascha; Küppers, Franko; Hartnagel, Hans
We report on high‐responsivity, fast near‐ultraviolet photodetectors based on bulk ZnSe employing a metal–semiconductor–metal structure with and without interdigital contacts. A very high responsivity of 2.42 and 4.44 A W−1 at 20 V bias voltage and high rejection rate of 7900 and 4810 for the light with a wavelength of 325 nm is obtained for photodetectors without and with interdigital contacts, which indicates an internal gain. The mechanism of internal gain is attributed to the impact ionization of ZnSe atoms under high internal electric field strength of 133 kV cm−1. Also a low dark current of ≈3.4 nA and high detectivity of ≈1.4 × 1011 cm Hz1/2 W−1 at a voltage of 20 V is achieved for the device with interdigital contacts at room temperature.
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, Hans
NEGATIVE DIFFERENTIAL RESISTANCE IN ZnO-BASED RESONANT TUNNELING DIODES
(IEEE, 2019) Sirkeli, Vadim; Vatavu, Sergiu; Yilmazoglu, Oktay; Preu, Sascha; Hartnagel, Hans
We present the results of a simulation study of resonant tunneling transport of non-polar m-plane ZnO/ZnMgO quantum structures with double and triple quantum barriers. It is found that in current density-voltage characteristics of such devices a region is present with negative differential resistance and this feature can be used for the generation of terahertz waves. The best performance at room temperature with output power of 912 μW @ 1 THz is derived for the non-polar m-plane ZnO/ZnMgO structures with triple quantum barriers and optimized design.
RECENT PROGRESS IN GaN-BASED DEVICES FOR TERAHERTZ TECHNOLOGY
(Springer Nature, 2020) Sirkeli, Vadim; Tiginyanu, Ion; Hartnagel, Hans
This 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.
THZ SPECTROSCOPY BY NARROW SPECTRAL EMISSION OF QUANTUM CASCADE LASERS FOR MEDICAL APPLICATIONS
(Institute of Electrical and Electronics Engineers, 2021) Hartnagel, Hans; Sirkeli, Vadim; Acedo, Pablo
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 not cause any harmful ionization in biological tissues. The terahertz radiation is strongly attenuated by water and is very sensitive to water content. The interest in terahertz imaging and spectroscopy of biologically related applications is increasing. This paper provides current status and recent advances in terahertz spectroscopy techniques in biological and medical applications. In particular, we report on our designs of THz Quantum-Cascade sources to identify cancerous tissues and other medical issues.
ZnO FOR INFRARED AND TERAHERTZ APPLICATIONS
(Elsevier, 2021) Sirkeli, Vadim; Hartnagel, Hans
This chapter reviews the recent progress of the theoretical and experimental studies of ZnO-based structures such as quantum cascade lasers, resonant-tunneling diodes, and quantum well detectors and their applications for infrared and terahertz spectral range. The role of spontaneous and piezoelectric polarization in polar ZnO-based structures and their impact on intersubband transitions and the performance of terahertz devices are discussed in detail. It is shown that ZnO-based compounds are promising materials for the fabrication of terahertz sources operating up to room temperature due to their unique properties such as their large bandgap, conduction band offset energy, and high longitudinal-optical phonon energy. Moreover, ZnO-based terahertz sources can cover the spectral region of 0.1–12 THz, which is very important for THz imaging and detection of explosive materials and medical spectroscopy applications, which could be not covered by conventional GaAs-based terahertz devices. In terms of the reported significant progress in the growth of nonpolar m-plane ZnO-based heterostructures and devices with low defect density, a wide perspective for the design and fabrication of high-power terahertz sources at room-temperature operation is now opened up.
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 room
ZnO-BASED TERAHERTZ QUANTUM CASCADE LASERS
(Elsevier, 2019) Sirkeli, Vadim; Hartnagel, Hans
High-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.
ZnSe-BASED SOLAR-BLIND ULTRAVIOLET PHOTODETECTORS WITH DIFFERENT SCHOTTKY CONTACT METALS
(CEP USM, 2021) Sirkeli, Vadim; Nedeoglo, Natalia; Nedeoglo, Dmitrii; Yilmazoglu, Oktay; Hajo, Ahid; Preu, Sascha; Kuppers, Franko; Hartnagel, Hans
We report on the selection of contact metallisations for ZnSe-based metal-semiconductor-metal ultraviolet photode- tectors. Our evaluation is based on Ni/Au, Cr/Au, and hybrid Ag-nanowire contacts. Low values of dark current of 0.32 nA, 0.82 nA and 1.64 nA at bias voltage of 15 V were achieved for photodetectors with Ag-NW, Ni/Au and Cr/Au interdigital contacts, respectively. The best performance of our ZnSe-based ultraviolet photodetectors is observed for Ni/Au interdigital contacts. This is due to the higher Schottky barrier height, which is equal to ~ 1.49 eV for Ni/Au contacts in comparison with ~ 1.26 eV for Cr/Au contacts. A very high responsivity of 5.40 AW-1 at bias voltage of 15 V for light with a wave- length of 325 nm is obtained for devices with Ni/Au interdigital contacts. Moreover, the maximum of photocurrent on/off ratio of 20342 and minimum of NEP of ~ 3 × 10-15 W Hz-1/2 at bias voltage of 15 V was achieved for this type of device.