Browsing by Author "Sirkeli, Vadim"

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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
IMPURITY DISTRIBUTION IN n-ZnSe CRYSTALS DOPED WITH Au
(2005) Nedeoglo, Dmitrii; Nedeoglo, Natalia; Sirkeli, Vadim
Hall effect, electric conductivity and mobility of charge carriers in n-ZnSe single crystals doped with Au are investigated in the temperature range from 77 to 300 K. Impurity distribution in the crystals was studied by a method of “layer-by-layer” etching of a sample surface. The model, which explains this distribution by a simultaneous diffusion of Zn and Au atoms during a long-term high-temperature annealing of as-grown crystals in Zn + Au melt, is proposed.
INTERACTION OF INTRINSIC DEFECTS WITH IMPURITIES IN AL DOPED ZnSn SINGLE CRYSTALS
(American Institute of Physics, 2007) Ivanova, G.N.; Nedeoglo, Dmitrii; Nedeoglo, Natalia; Sirkeli, Vadim
We report on the results of a complex study of electrical (77−300 K) and luminescence (10−300 K) properties of 𝑛-ZnSe single crystals annealed in a Zn melt containing Al impurity at concentrations ranging from 0.1 to 80 at. %. It was established that Al atoms form donor centers only at a low impurity concentration (≤0.5 at. %). The increase of the amount of Al atoms in the crystal results in the formation of (VZnAlZn) associative acceptor centers leading to the self-compensation of the shallow Al donor impurity. This process is accompanied by the emergence and development of a self-activated luminescence band associated with the (VZnAlZn) acceptor centers. We show that further increase of the Al content in the melt (≥10 at. %) leads to the dissociation of the acceptor complexes and to a recurrent donor doping effect. The photoluminescence spectra of such crystals are dominated by activated luminescence via the (CuZnVSeCu𝑖) and (CuZnAlZn) associative centers.
MAGNETIC AND LUMINESCENT PROPERTIES OF IRON-DOPED ZnSe CRYSTALS
(Elsevier, 2010) Kulyuk, Leonid; Nedeoglo, Dmitrii; Nedeoglo, Natalia; Radevici, Ivan; Sirkeli, Vadim; Sushkevich, Konstantin
Magnetic and luminescent properties of ZnSe crystals doped with Fe by various methods are studied. It is established that Fe impurity is responsible for photoluminescence (PL) bands at 980, 1320, 1450 nm and quenches PL band at 630–645 nm. It is found that magnetic properties of ZnSe:Fe crystals are sensitive to the doping method. At low fields, two magnetic subsystems may be observed for the samples doped with Fe during the growth process—weak paramagnetic subsystem and antiferromagnetic subsystem with TС=–130 K. For the samples doped with Fe by high-temperature annealing in Zn melt, few magnetic subsystems may be distinguished, however, the magnetic properties are typical for spin glasses with the transition temperature Tsg=(45–50) K.
MAGNETIC AND LUMINESCENT PROPERTIES OF NICKEL-DOPED ZnSe CRYSTALS
(Elsevier, 2015) Sirkeli, Vadim; Radevici, Ivan; Sushkevich, Konstantin; Nedeoglo, Natalia; Nedeoglo, Dmitrii
Magnetic and photoluminescent properties of nickel-doped ZnSe crystals with impurity concentrations varied by changing the Ni amount in the source material from 0.001 to 0.50 at.% are studied in 5–300 K temperature range. Investigation of magnetic properties shows that Ni impurity in ZnSe forms isolated paramagnetic centers and probability of Ni–Ni pairs formation is negligible due to low Ni concentration in the samples. The contribution of Ni impurity to edge emission and its influence on infra-red emission are discussed. It is found that complete concentration quenching of luminescence within all studied spectral range is observed starting with Ni concentration of 0.50 at.%.
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.
PHOTOLUMINESCENCE OF n-ZnSe SINGLE CRYSTALS DOPED WITH IODINE BY VAPOUR PHASE DIFFUSION
(2006) Avdonin, A.N.; Nedeoglo, Dmitrii; Nedeoglo, Natalia; Sirkeli, Vadim; Ivanova, G.N.
Photoluminescence spectra of n-ZnSe single crystals doped with iodine are investigated in the temperature range from 83 to 300 K. It is shown that the edge PL band is formed by overlapping of the PL lines attributed to ISe donor-bound ecxitons (I2I) and VZn acceptorbound excitons (D1I). A model of radiation mechanisms, which explain the redistribution of the edge and long-wave PL bands intensities with increasing doping level of the samples, is proposed.
PHOTOLUMINESCENCE STUDY OF ZnO NANOSTRUCTURES GROWN ON SILICON BY MOCVD
(Elsevier, 2012) Sirkeli, Vadim; Nedeoglo, Dmitrii
ZnO nanostructures with a size ranging from 20 to 100 nm were successfully deposited on (1 0 0)-Si substrates at different temperatures (500–800 °C) using MOCVD. It could be confirmed that the size of ZnO nanostructures decreased with increasing growth temperature. From photoluminescence (PL) studies it was found, that intensive band-edge PL of ZnO nanostructures consists of emission lines with maxima at 368.6 nm, 370.1 nm, 373.7 nm, 383.9 nm, 391.7 nm, 400.7 nm and 412 nm. These lines can be dedicated to free excitons and impurity donor-bound excitons, where hydrogen acts as donor impurity with an activation energy of about 65 meV. A UV shift of the band-edge PL line with increasing growth temperature of ZnO nanostructures was observed as a result of the quantum confinement effect. The results suggest that an increase of growth temperature leads to increased band-edge PL intensity. Moreover, the ratio of band-edge PL intensity to green- (red-) band intensity also increases, indicating better crystalline quality of ZnO nanostructures with increasing growth temperature.
PROPOSAL FOR A MONOLITHIC BROADBAND TERAHERTZ QUANTUM CASCADE LASER ARRAY TAILORED TO DETECTION OF EXPLOSIVE MATERIALS
(American Scientific Publishers, 2018) Sirkeli, Vadim; Yilmazoglu, Oktay
Since most tunable THz sources produce only small power levels, we propose for routine evaluation of dangerous materials to employ a chain of quantum cascade THz generators, where each of them addresses a specific spectroscopic line of the relevant identifying spectrum. Therefore, we present the design, operating principle and performance of a room-temperature monolithic broadband terahertz (THz) source for applications of THz imaging and detection of explosive materials such as TNT, RDX, PETN and HMX. The suggested terahertz source is a 20-element array of quantum cascade lasers (QCLs) emitting at discrete frequencies from 0.85 to 4.74 THz. The layer structure of each individual THz QCL is based on a two-well design scheme with variable barrier heights and resonant-phonon depopulation of the lower laser state. The tailoring of emission frequency of individual THz QCLs in the laser array was made by varying the constituent epilayers' width and doping level of the injector well. We found that the peak optical gain of the discrete THz QCLs is increased with increasing tailored THz emission frequency. The detection of the transmitted line can be done by THz Schottky diodes after relevant narrow-band filters. The other detector concept could be quantum cascading, where its narrow-band filter property allows the detection of the relevant THz line. This system is intended for routine security testing, where speed and reliability are required.
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, Petriina
Hall coefficient, electrical conductivity, and electron mobility are investigated for n-ZnSe:Yb single crystals with high concentration of electrically active background impurities
QUANTUM NANOSTRUCTURES FOR TERAHERTZ DEVICES AND APPLICATIONS
(2019) Sirkeli, Vadim
erahertz (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.
RECENT ADVANCES IN TERAHERTZ TECHNOLOGY FOR SECURITY AND BIOMEDICAL APPLICATIONS
(Universitatea de Stat din Tiraspol, 2021) Sirkeli, Vadim
Terahertz 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.
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.
RECENT PROGRESS IN WIDE BANDGAP SEMICONDUCTORS-BASED DEVICES FOR TERAHERTZ APPLICATIONS
(CEP USM, 2020) Sirkeli, Vadim
RESONANT TUNNELING AND QUANTUM CASCADING FOR OPTIMUM ROOM-TEMPERATURE GENERATION OF THz SIGNALS
(Institute of Electrical and Electronics Engineers, 2017) Sirkeli, Vadim; Yilmazoglu, Oktay
We report on the results of a numerical study of quantum transport in ZnSe-based resonant-tunneling diodes (RTDs) and quantum cascade oscillators (QCOs) with fixed and unequal barrier heights. It is found that the negative differential resistance exists up to room temperature in the current-voltage characteristics of the RTD and QCO devices with unequal barrier heights. Further, we demonstrate that QCOs with unequal barrier heights have a better frequency and power performance characteristics compared with RTDs and are more beneficial for high-power terahertz generation at room temperature. For the best QCO device with 100 periods of quantum cascading, a maximum output power of ~7-9 μW for the operating frequency range from 0.1 to ~6 THz at room temperature was achieved.
SHALLOW DONOR STATES INDUCED IN ZNSE:AU SINGLE CRYSTALS BY LATTICE DEFORMATION
(American Institute of Physics, 2008) Nedeoglo, Natalia; Nedeoglo, Dmitrii; Laiho, R.; Sirkeli, Vadim; Lähderanta, E.
Photoluminescence (PL) spectra are investigated in n-ZnSe single crystals at different temperatures from 4.4 to 300 K immediately after doping with Au from melt of Se+Au or Zn+Au and after storage of the doped samples for 4 years in the dark under normal room conditions. Due to the formation of Aui interstitial donors in the n-ZnSe:Se:Au crystals with time, the origin of the near band edge PL changes from acceptor-bound to donor-bound excitons. Taking into account the results of PL characterization, we proposed that the Aui donors are generated by displacement of Au ions from regular lattice sites to interstitial sites with the help of lattice deformation forces. Transport measurements show dramatic increase in the electrical conductivity and the free electron concentration after storage of the n-ZnSe:Zn:Au crystals, thus confirming the proposed model.
TERAHERTZ RESONANT TUNNELING DIODES BASED ON GaN/AlGaN STRUCTURES
(CEP USM, 2022-11-10) Sirkeli, Vadim