Andrey Naumov | National Academy of Sciences of Ukraine (original) (raw)
Papers by Andrey Naumov
Article, Oct 19, 2017
We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20- pe... more We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20- period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed.
Article, Feb 27, 2017
We studied space-charge-distribution phenomena in planar GaN nanowires and nanoribbons (NRs). The... more We studied space-charge-distribution phenomena in planar GaN nanowires and nanoribbons (NRs). The results obtained at low voltages demonstrate that the electron concentration changes not only at the edges of the NR, but also in the middle part of the NR. The effect is stronger with decreasing NR width. Moreover, the spatial separation of the positive and negative charges results in electric-field patterns outside the NR. This remarkable feature of electrostatic fields outside the NR may be even stronger in 2D material structures. For larger voltages the space-charge-limited current (SCLC) effect determines the main mechanism of transport in the NR samples. The onset of the SCLC effect clearly correlates with the NR width. The results are confirmed by noise spectroscopy studies of the NR transport. We found that the noise increases with decreasing NR width and the shape of the spectra changes with voltage increase with a tendency toward slope (3/2), reflecting diffusion processes due to the SCLC effect. At higher voltages noise decreases as a result of changes in the scattering mechanisms. We suggest that the features of the electric current and noise found in the NRs are of general character and will have an impact on the development of NR-based devices.
Article, Jan 10, 2017
We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low ... more We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low voltages provide us with the estimates of the depletion effects in the NWs. For larger voltages, we observe the space-charge limited current (SCLC) effect. The onset of the effect clearly correlates with the NW width. For narrow NWs, the mature SCLC regime was achieved. This effect has great impact on fluctuation characteristics of studied NWs. At low voltages, we found that the normalized noise level increases with decreasing NW width. In the SCLC regime, a further increase in the normalized noise intensity (up to 10 4 times) was observed, as well as a change in the shape of the spectra with a tendency towards slope −3/2. We suggest that the features of the electric current and noise found in the NWs are of a general character and will have an impact on the development of NW-based devices.
Article, Feb 1, 2016
The influence of strain and barrier/well thickness ratio on recombination processes in multi-quan... more The influence of strain and barrier/well thickness ratio on recombination processes in multi-quantum well (MQW) Al0.1Ga0.9N/GaN structures was investigated using X-ray diffraction and Raman and photoluminescence spectroscopies. The deformation state of the wells and barriers was determined. In addition, the value of the polarization fields, the density of polarization charges, and the positions of energy levels for optical transitions within the quantum wells were calculated. It was established that compressive strain in the buffer layer as well as in the layers of the MQWs with respect to the buffer layer lead to the piezoelectric fields having equal sign in the well and the barrier. As a result, the recombination of donor–acceptor pairs dominates over transitions between electron and hole states in the quantum well.
Article, Dec 3, 2015
The effect of self-heating on the transport characteristics and electronic properties of transist... more The effect of self-heating on the transport characteristics and electronic properties of transistor AlGaN/GaN heterostructures was investigated. The electrical, micro-Raman and photoluminescence techniques were used for temperature estimations for transistor structures under electrical load. The thermal resistance of these structures has been calculated to obtain the temperature of conducting channel heating from the current-voltage characteristics. The differences in the obtained temperature data from applied techniques have been analyzed.
Conference Paper: 73rd Annual Scientific Conference of Univ. Latvia, Section: FOTONIKA-LV FP7-REGPOT-CT-2011-285912 Project - The Third Year Scientific Outcomes, 6-7 Feb 2015, Riga, Latvia, Book of Abstracts, Ed. A. Atvars, V. Beldavs, A. Ubelis, Riga Photonics Centre, Univ. Latvia, Feb 6, 2015
Possible approaches to bio-sensing and diagnosis of environmental stress-induced responses in mit... more Possible approaches to bio-sensing and diagnosis of environmental stress-induced responses in mitochondrial subcellular structures by biomarkers using (i) fluorescent optical microscopy and flow cytometry or (ii) bioelectronic sensors based on field-effect transistors with semiconductor nanowires and microwave whispering-gallery-mode dielectric resonators with microfluidic channels are discussed and analyzes in terms of their use in biological studies.
References
[1] N. Kandaurova, et al., Peculiarities of optical imaging of mitochondrial organelles in biological studies, in Optics and High Technology Material Science, Taras Shevchenko Nat. Univ. Kyiv, DP.7 p.213 (2013)
[2] M. Landrum M. et al., BODIPY probes to study peroxisome dynamics in vivo, Plant J., Vol.62, pp.529-538 (2010)
[3] A. Poghossian, M. J. Schoning, Label-free sensing of biomolecules with field-effect devices for clinical applications, Electroanalysis, Vol.26, pp.1197-1213 (2014)
Article, Sep 5, 2013
The temperature dependences of the contact resistance ρc (T) of ohmic Pd-Ti-Pd-Au contacts to n-G... more The temperature dependences of the contact resistance ρc (T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρc (T) for both contacts contain portions of exponential decrease ρc (T) and very weak dependence ρc (T) at higher temperatures. Furthermore, a plateau portion ρc (T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρc (T) for ohmic contacts to n-GaN and n-AlN are proposed.
Article, Dec 8, 2012
We studied temperature dependences of resistivity of Pd-Ti-Pd-Au ohmic contacts to wide-gap semic... more We studied temperature dependences of resistivity of Pd-Ti-Pd-Au ohmic contacts to wide-gap semiconductors n-GaN and n-AlN with high dislocation density. Both curves have portions of exponential decrease, as well as those with very slight dependence at higher temperatures. Besides, the Au-Pd-Ti-Pd-n-GaN contacts have a portion of flattening out in the low-temperature region. This portion appears only after rapid thermal annealing (RTA). In principle, its appearance may be due to previous heavy doping of the near-contact region with a shallow donor impurity as well as doping in the course of contact formation as a result of RTA if the contact-forming layer involves a material whose atoms serve as shallow donors in the III-N compounds. The obtained dependences cannot be explained by the existing mechanisms of current flow. We propose other mechanisms explaining the experimental curves for ohmic contacts to n-GaN and n-AlN.
Article, Apr 1, 2009
We report the results of direct measurements and a theoretical investigation of the in-plane effe... more We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2+-0.01) m0.
Article, Apr 1, 2009
This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. ... more This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. We compared the overheating temperatures in the transmission line model patterned devices during normal operation and analyze their dependence on the buffer thickness in order to optimize the thermal budget of the structures. It is demonstrated that noise spectra can be used to monitor the transport mechanisms and determine the activation energy of the traps. Small-dose gamma irradiation is used as an effective treatment for improving the structural properties. Based on cathodoluminescence and X-ray diffraction spectra shape, we explain the improvement as a result of the relaxation of elastic strains and structural-impurity ordering in the AlGaN barrier layer under irradiation. An irreversible improvement in mobility of the electrons in the channel, controlled by a proper dose of gamma radiation treatment, at considerably reduced self-heating is promising to achieve novel quantum transport regimes on the nanoscale.
Article, Jan 8, 2009
In this paper we discuss the effect of dissipated power on the characteristic time of noise spect... more In this paper we discuss the effect of dissipated power on the characteristic time of noise spectra transformation investigated in wide-bandgap AlGaN/GaN transistor structures with long channels. It is found that the characteristic time as a function of temperature demonstrates an exponential dependence with definite activation energy. Obtained results are explained based on the developed model of non-equilibrium fluctuations of the sample resistance. These fluctuations cause a local overheating due to local change of resistance. In the long channels these fluctuations may create overheating regions with two different self-heating temperatures, leading to different activation dependences, observed in the experiment.
Article, Oct 21, 2008
This paper reports on the results of the experimental and numerical investigation into the self-h... more This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 × 106 rad of 60Co gamma rays.
Article, Feb 7, 2007
Photoluminescence PL in modulation-doped and nominally undoped AlxGa1−xN/GaN heterostructures was... more Photoluminescence PL in modulation-doped and nominally undoped AlxGa1−xN/GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3 - 3.46 eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped AlxGa1−xN/GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7 - 3.3 eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in AlxGa1−xN/GaN heterostructures introduced at the modulation doping of the AlxGa1−xN barrier layer.
Article, Aug 18, 2006
Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and op... more Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and optical spectral analysis is carried out. The layers are grown by metalloorganic gas-phase epitaxy on (0001)-oriented single crystal sapphire wafers. The components of strains and the density of dislocations are deter-mined. The effects of strains and dislocations on the photoluminescence intensity and spectra are studied. The results allow better understanding of the nature and mechanisms of the formation of defects in the epitaxial AlGaN/GaN heterostructures.
Article, May 17, 2006
We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling di... more We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling diodes (RTDs) using steady-state current-voltage and capacitance-voltage (C-V) characteristics in a wide frequency range with 2 kHz steps. The C-V characteristics of a double-barrier RTD show different behaviour under forward and reverse polarities and a strong dependence on frequency. The monotonous growth of capacitance at forward bias was registered, while a more complicated dependence was observed at reverse voltages. In order to analyse this dependence, a self-consistent calculation of the potential profile of the structure was performed taking into account polarization effects at the AlN/GaN interfaces. The peculiarities are analysed in the model of possible charge trapping at the interface states.
Article, Jul 1, 2004
Present paper studies double barrier resonant tunnelling diodes (DB-RTD) based on GaN/AlGaN heter... more Present paper studies double barrier resonant tunnelling diodes (DB-RTD) based on GaN/AlGaN heterostructures, grown by plasma-assisted molecular beam epitaxy (PA-MBE). Tunnel (current-voltage, IV) and capacitance (capacitance-voltage, CV) spectroscopy measurements were performed at the temperature range from 4.2 to 300 K. It has been found that measured characteristics of DB-RTD have complex nonlinear behavior and reveal the current discontinuities of IV curves. The features can be explained by the existence of polarization fields and interface defects. These effects strongly influence on the potential profile of the DB-RTD heterostructures. To understand physics of the processes, numerical simulations of the given structures, using a model based on real-time Greens functions, have been performed. Comparative analysis of experimental and numerical data showed that the current instability and nonlinearity of characteristics of the nitride based DB-RTD can be connected with trapping the electrons onto interfacial and dislocation states in these heterostructures.
Article, Oct 19, 2017
We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20- pe... more We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20- period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed.
Article, Feb 27, 2017
We studied space-charge-distribution phenomena in planar GaN nanowires and nanoribbons (NRs). The... more We studied space-charge-distribution phenomena in planar GaN nanowires and nanoribbons (NRs). The results obtained at low voltages demonstrate that the electron concentration changes not only at the edges of the NR, but also in the middle part of the NR. The effect is stronger with decreasing NR width. Moreover, the spatial separation of the positive and negative charges results in electric-field patterns outside the NR. This remarkable feature of electrostatic fields outside the NR may be even stronger in 2D material structures. For larger voltages the space-charge-limited current (SCLC) effect determines the main mechanism of transport in the NR samples. The onset of the SCLC effect clearly correlates with the NR width. The results are confirmed by noise spectroscopy studies of the NR transport. We found that the noise increases with decreasing NR width and the shape of the spectra changes with voltage increase with a tendency toward slope (3/2), reflecting diffusion processes due to the SCLC effect. At higher voltages noise decreases as a result of changes in the scattering mechanisms. We suggest that the features of the electric current and noise found in the NRs are of general character and will have an impact on the development of NR-based devices.
Article, Jan 10, 2017
We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low ... more We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low voltages provide us with the estimates of the depletion effects in the NWs. For larger voltages, we observe the space-charge limited current (SCLC) effect. The onset of the effect clearly correlates with the NW width. For narrow NWs, the mature SCLC regime was achieved. This effect has great impact on fluctuation characteristics of studied NWs. At low voltages, we found that the normalized noise level increases with decreasing NW width. In the SCLC regime, a further increase in the normalized noise intensity (up to 10 4 times) was observed, as well as a change in the shape of the spectra with a tendency towards slope −3/2. We suggest that the features of the electric current and noise found in the NWs are of a general character and will have an impact on the development of NW-based devices.
Article, Feb 1, 2016
The influence of strain and barrier/well thickness ratio on recombination processes in multi-quan... more The influence of strain and barrier/well thickness ratio on recombination processes in multi-quantum well (MQW) Al0.1Ga0.9N/GaN structures was investigated using X-ray diffraction and Raman and photoluminescence spectroscopies. The deformation state of the wells and barriers was determined. In addition, the value of the polarization fields, the density of polarization charges, and the positions of energy levels for optical transitions within the quantum wells were calculated. It was established that compressive strain in the buffer layer as well as in the layers of the MQWs with respect to the buffer layer lead to the piezoelectric fields having equal sign in the well and the barrier. As a result, the recombination of donor–acceptor pairs dominates over transitions between electron and hole states in the quantum well.
Article, Dec 3, 2015
The effect of self-heating on the transport characteristics and electronic properties of transist... more The effect of self-heating on the transport characteristics and electronic properties of transistor AlGaN/GaN heterostructures was investigated. The electrical, micro-Raman and photoluminescence techniques were used for temperature estimations for transistor structures under electrical load. The thermal resistance of these structures has been calculated to obtain the temperature of conducting channel heating from the current-voltage characteristics. The differences in the obtained temperature data from applied techniques have been analyzed.
Conference Paper: 73rd Annual Scientific Conference of Univ. Latvia, Section: FOTONIKA-LV FP7-REGPOT-CT-2011-285912 Project - The Third Year Scientific Outcomes, 6-7 Feb 2015, Riga, Latvia, Book of Abstracts, Ed. A. Atvars, V. Beldavs, A. Ubelis, Riga Photonics Centre, Univ. Latvia, Feb 6, 2015
Possible approaches to bio-sensing and diagnosis of environmental stress-induced responses in mit... more Possible approaches to bio-sensing and diagnosis of environmental stress-induced responses in mitochondrial subcellular structures by biomarkers using (i) fluorescent optical microscopy and flow cytometry or (ii) bioelectronic sensors based on field-effect transistors with semiconductor nanowires and microwave whispering-gallery-mode dielectric resonators with microfluidic channels are discussed and analyzes in terms of their use in biological studies.
References
[1] N. Kandaurova, et al., Peculiarities of optical imaging of mitochondrial organelles in biological studies, in Optics and High Technology Material Science, Taras Shevchenko Nat. Univ. Kyiv, DP.7 p.213 (2013)
[2] M. Landrum M. et al., BODIPY probes to study peroxisome dynamics in vivo, Plant J., Vol.62, pp.529-538 (2010)
[3] A. Poghossian, M. J. Schoning, Label-free sensing of biomolecules with field-effect devices for clinical applications, Electroanalysis, Vol.26, pp.1197-1213 (2014)
Article, Sep 5, 2013
The temperature dependences of the contact resistance ρc (T) of ohmic Pd-Ti-Pd-Au contacts to n-G... more The temperature dependences of the contact resistance ρc (T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρc (T) for both contacts contain portions of exponential decrease ρc (T) and very weak dependence ρc (T) at higher temperatures. Furthermore, a plateau portion ρc (T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρc (T) for ohmic contacts to n-GaN and n-AlN are proposed.
Article, Dec 8, 2012
We studied temperature dependences of resistivity of Pd-Ti-Pd-Au ohmic contacts to wide-gap semic... more We studied temperature dependences of resistivity of Pd-Ti-Pd-Au ohmic contacts to wide-gap semiconductors n-GaN and n-AlN with high dislocation density. Both curves have portions of exponential decrease, as well as those with very slight dependence at higher temperatures. Besides, the Au-Pd-Ti-Pd-n-GaN contacts have a portion of flattening out in the low-temperature region. This portion appears only after rapid thermal annealing (RTA). In principle, its appearance may be due to previous heavy doping of the near-contact region with a shallow donor impurity as well as doping in the course of contact formation as a result of RTA if the contact-forming layer involves a material whose atoms serve as shallow donors in the III-N compounds. The obtained dependences cannot be explained by the existing mechanisms of current flow. We propose other mechanisms explaining the experimental curves for ohmic contacts to n-GaN and n-AlN.
Article, Apr 1, 2009
We report the results of direct measurements and a theoretical investigation of the in-plane effe... more We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2+-0.01) m0.
Article, Apr 1, 2009
This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. ... more This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. We compared the overheating temperatures in the transmission line model patterned devices during normal operation and analyze their dependence on the buffer thickness in order to optimize the thermal budget of the structures. It is demonstrated that noise spectra can be used to monitor the transport mechanisms and determine the activation energy of the traps. Small-dose gamma irradiation is used as an effective treatment for improving the structural properties. Based on cathodoluminescence and X-ray diffraction spectra shape, we explain the improvement as a result of the relaxation of elastic strains and structural-impurity ordering in the AlGaN barrier layer under irradiation. An irreversible improvement in mobility of the electrons in the channel, controlled by a proper dose of gamma radiation treatment, at considerably reduced self-heating is promising to achieve novel quantum transport regimes on the nanoscale.
Article, Jan 8, 2009
In this paper we discuss the effect of dissipated power on the characteristic time of noise spect... more In this paper we discuss the effect of dissipated power on the characteristic time of noise spectra transformation investigated in wide-bandgap AlGaN/GaN transistor structures with long channels. It is found that the characteristic time as a function of temperature demonstrates an exponential dependence with definite activation energy. Obtained results are explained based on the developed model of non-equilibrium fluctuations of the sample resistance. These fluctuations cause a local overheating due to local change of resistance. In the long channels these fluctuations may create overheating regions with two different self-heating temperatures, leading to different activation dependences, observed in the experiment.
Article, Oct 21, 2008
This paper reports on the results of the experimental and numerical investigation into the self-h... more This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 × 106 rad of 60Co gamma rays.
Article, Feb 7, 2007
Photoluminescence PL in modulation-doped and nominally undoped AlxGa1−xN/GaN heterostructures was... more Photoluminescence PL in modulation-doped and nominally undoped AlxGa1−xN/GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3 - 3.46 eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped AlxGa1−xN/GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7 - 3.3 eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in AlxGa1−xN/GaN heterostructures introduced at the modulation doping of the AlxGa1−xN barrier layer.
Article, Aug 18, 2006
Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and op... more Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and optical spectral analysis is carried out. The layers are grown by metalloorganic gas-phase epitaxy on (0001)-oriented single crystal sapphire wafers. The components of strains and the density of dislocations are deter-mined. The effects of strains and dislocations on the photoluminescence intensity and spectra are studied. The results allow better understanding of the nature and mechanisms of the formation of defects in the epitaxial AlGaN/GaN heterostructures.
Article, May 17, 2006
We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling di... more We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling diodes (RTDs) using steady-state current-voltage and capacitance-voltage (C-V) characteristics in a wide frequency range with 2 kHz steps. The C-V characteristics of a double-barrier RTD show different behaviour under forward and reverse polarities and a strong dependence on frequency. The monotonous growth of capacitance at forward bias was registered, while a more complicated dependence was observed at reverse voltages. In order to analyse this dependence, a self-consistent calculation of the potential profile of the structure was performed taking into account polarization effects at the AlN/GaN interfaces. The peculiarities are analysed in the model of possible charge trapping at the interface states.
Article, Jul 1, 2004
Present paper studies double barrier resonant tunnelling diodes (DB-RTD) based on GaN/AlGaN heter... more Present paper studies double barrier resonant tunnelling diodes (DB-RTD) based on GaN/AlGaN heterostructures, grown by plasma-assisted molecular beam epitaxy (PA-MBE). Tunnel (current-voltage, IV) and capacitance (capacitance-voltage, CV) spectroscopy measurements were performed at the temperature range from 4.2 to 300 K. It has been found that measured characteristics of DB-RTD have complex nonlinear behavior and reveal the current discontinuities of IV curves. The features can be explained by the existence of polarization fields and interface defects. These effects strongly influence on the potential profile of the DB-RTD heterostructures. To understand physics of the processes, numerical simulations of the given structures, using a model based on real-time Greens functions, have been performed. Comparative analysis of experimental and numerical data showed that the current instability and nonlinearity of characteristics of the nitride based DB-RTD can be connected with trapping the electrons onto interfacial and dislocation states in these heterostructures.