V. M. Polyakov

Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg, Baden-Württemberg, Germany

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Publications (52)89.95 Total impact

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    ABSTRACT: In this work we discuss the influence of the donor-like surface state density (SSD) on leakage currents and the breakdown voltages of AlGaN/GaN heterostructure field-effect transistors (HFET) at high temperature reverse bias (HTRB) step stress. A method to extract charges at the surface by high voltage capacitance voltage (HV–CV) profiling of the gate–drain diode of a HFET is presented. Two samples with different surface passivation are compared. The SSD of the first sample is found to be similar to the polarization charge, whereas it is elevated by a factor of three on the second sample. The influence of the SSD on the electric field is investigated with electroluminescence (EL). The elevated SSD of the second sample engenders severe deficiencies in robustness found in the HTRB. The stress data, the simulation model and the images of EL indicate that the catastrophic failure arises in the dielectric underneath the gate field plate (GFP).
    Microelectronics Reliability 12/2014; · 1.21 Impact Factor
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    ABSTRACT: We report on electroluminescence (EL) emission from AlGaN/GaN high electron mobility transistors (HEMTs). Intensity maxima at the drain-side edge of the gate foot and at the drain-side edge of the gate field plate are observed. To relate the EL intensity profile to the electric field along the channel, 2D device simulations have been performed at different drain biases. The dependences of both EL maxima on the electric field reveal a threshold which closely correlates with the electric field strength at which a transfer of conduction band electrons from the zone centre minimum to satellite valleys sets in. We further analyze the dependence of the EL spectra on the drain voltage. The obtained results strongly suggest that the EL emission observed in AlGaN/GaN HEMTs is dominated by radiative inter-valley electron transitions.
    Semiconductor Science and Technology 10/2012; 27(12):125003. · 2.21 Impact Factor
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    ABSTRACT: To extend the tuning capabilities of radio frequency devices, coupled microelectromechanical systems are often employed. In this letter, we demonstrate piezoelectrically actuated, electrically tuneable resonator systems based on coupled micromechanical oscillators operating in a flexural vibration mode. The substantial enhancement in electrostatic coupling was achieved due to the implementation of lateral nanogaps of 100-200 nm between single resonator bars. This allows for resonator synchronization and precise system frequency tuning by over a factor of two, relative to its initial value. Additionally, a simple electro-mechanical model has been developed to describe the dynamic behavior of the electrostatically coupled oscillators.
    Applied Physics Letters 03/2012; 100(12). · 3.52 Impact Factor
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    ABSTRACT: The formation of two-dimensional carrier gases in gated GaN/AlGaN/GaN heterostructures is investigated theoretically. It is shown that under certain conditions a two-dimensional hole gas at the upper GaN/AlGaN interface can be formed in addition to the two-dimensional electron gas at the lower AlGaN/GaN interface. For the calculations, a Schrödinger-Poisson solver and a simple analytical model developed in the present work are used. Conditions for the formation of a two-dimensional hole gas are elaborated. It is shown that once a two-dimensional hole gas is created, it shields the coexisting two-dimensional electron gas which will result in a diminishing effect of the gate voltage on the two-dimensional electron gas.
    Journal of Applied Physics 02/2012; 111(4). · 2.19 Impact Factor
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    ABSTRACT: The effect of RF stress at 10 GHz and DC stress on AlGaN/GaN HEMTs have been investigated by comparing static and transient characteristics before and after stress. It was found that the threshold voltage shifts in both tests significantly to the negative. A defect level of 0.44 eV was detected during Id-trapping analysis. Using the experimental trap data and simulating different locations of traps in the device it was established that the defective region is extended throughout the gate region. Quantitative approximations of the trap density suggest an extension of the traps into the barrier layer with a concentration of approximately 1018 cm-3.
    Reliability Physics Symposium (IRPS), 2012 IEEE International; 01/2012
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    ABSTRACT: The influence of the electric field on the reliability of AlGaN/GaN HEMTs is investigated in this work. We first demonstrate that at a certain electric field strength at the gate edge the gate characteristics of the device changes. This degradation is irreversible and is strongly influenced by growth parameter. A drain-voltage step-stress method is applied to the devices for investigating different layouts, and a consequent application enabled us to assign parameters mitigating the peak field strength and improve reliability.
    Microelectronics Reliability 02/2011; 51(2):224-228. · 1.21 Impact Factor
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    ABSTRACT: Electrical properties of AlxGa1-xN/GaN heterostructures with an Al content below 15% and carrier concentrations as low as 1.0 × 1012 cm-2 were investigated by Hall effect measurements and capacitance-voltage profiling. The nominally undoped GaN capped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The threshold voltage of transistor devices follows the trend already found for high Al-containing structures, which are described by a model indicating a surface potential independent of Al content. Photoreflectance spectroscopy confirms the results for as-grown heterostructures. The Hall effect measured on the as-grown samples, however, shows a stronger decrease in carrier concentration than expected from the effect of polarization and constant surface potential. In contrast, Hall effect data determined on samples with Ni Schottky contacts and capacitance-voltage profiling on as-grown samples yield the expected behavior, with surface potentials of 0.86 eV and 0.94 eV, respectively. The inconsistency is eliminated by describing the results of the Hall effect on as-grown samples by a two-carrier model. Self-consistent Schrödinger-Poisson calculations support these considerations if we take into account a transition range at the AlxGa1-xN/GaN interface.
    Journal of Applied Physics 01/2011; 109. · 2.19 Impact Factor
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    ABSTRACT: For high frequency field effect transistors, one of the promising approaches is to grow a very thin (≤10 nm) InN channel pseudomorphically with low defect density between low lattice mismatched InGaN layers. The present work provides a comprehensive analysis of such structures by varying width of the InN well. Both experimental and theoretical approaches have been applied in order to optimize the InN potential well structure for a transistor operation. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 10/2010; 8(2):485 - 487.
  • V.M. Polyakov, V. Cimalla, V. Lebedev, F. Schwierz
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    ABSTRACT: We theoretically investigate the influence of n-type doping on the terahertz (THz) electric field emission from unbiased c-plane InN surfaces using the ensemble Monte Carlo (MC) method. It is shown that the increase of n-type doping has twofold effect on the THz surface emission. The detrimental effect of electron drift mobility decrease is compensated by doping-derived electrons constructively contributing to the total dynamic dipole responsible for a generation of the THz electric field pulse emission from InN surface.
    physica status solidi (a) 05/2010; · 1.21 Impact Factor
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    A. Knübel, V. M. Polyakov, L. Kirste, R. Aidam
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    ABSTRACT: InxGa1−xN films with 0.4 ≤ x ≤ 1 are analyzed using electrolyte-based capacitance-voltage technique. In-rich InxGa1−xN for x>0.4 samples exhibit a strong surface electron accumulation. At x = 0.4, the Fermi level at the surface is pinned to the conduction band edge indicating a crossover from surface accumulation to depletion. The measured Mott–Schottky plots are fitted using a model based on a Schrödinger–Poisson solver. By comparing the measured data to the fitting results, we conclude that a subsurface layer of ∼ 15 nm thickness with remarkably lower donor defect concentration is formed during the growth of InGaN films.
    Applied Physics Letters 02/2010; 96(8):082106-082106-3. · 3.52 Impact Factor
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    ABSTRACT: We studied InN (0001) grown by plasma assisted molecular beam epitaxy (PAMBE) with high-resolution electron energy-loss spectroscopy (HREELS) after vacuum transfer thus avoiding any further surface preparation. We were able to detect for the first time besides the Fuchs-Kliewer surface phonon ω0 two plasmarons ω– and ω+, which originate from the coupling between surface optical phonons ωSO and plasma oscillations of free electrons resulting from the surface accumulation layer and/or the bulk conduction band. Using dielectric theory we get new insight into the space charge regime of InN grown by MBE. We choose an approach that reaches far beyond the accumulation layer regime. Using a very wide primary beam energy regime from 1 eV up to 200 eV, the depth of information can be varied from extremely surface sensitive to bulk sensitive. Best agreement has been obtained for a downward band bending of 0.9 eV and a peak accumulation layer density of 6.5 × 1019 cm-3 at 2 nm beneath the surface with an unintentional doping of the bulk of 1 × 1019 cm-3. The sheet density of the ionized surface states is determined to Nss = 1.8 × 1013 cm-2 from the self-consistent calculation of the conduction band profile. The presented results provide important information for future InN-based devices. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2010; 7(2):173 - 176.
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    ABSTRACT: The vertical design of indium nitride field effect transistors is investigated by numerical simulation. To this end, the Schrödinger equations for electrons and holes and Poisson's equation are solved self-consistently. It is shown that in several layer sequences simultaneously two-dimensional electron and hole gases are formed in the InN channel. It is demonstrated that because of the high unintentional n-type doping only thin InN layers are useful for proper transistor operation. Strain in the InN layer leads to the formation of parasitic hole channels which can dramatically deteriorate transistor characteristics. Finally it is shown that thin relaxed InN channels on GaN or AlInN buffers are a viable option for InN transistors.
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    ABSTRACT: The influence of the Al content on the mobility of the two-dimensional electron gas (2DEG) in GaN/AlxGa1-xN/GaN heterostructures is studied by employing the ensemble Monte Carlo method. Using two interface polarization charge models, we calculate the room temperature low-field mobility at different Al compositions of the barrier layer ranging from 8% up to 35%. All relevant scattering mechanisms are considered to provide a quantitative description of the measured mobilities. We show that 2DEG transport in the heterostructures is mostly affected by dislocation scattering for all Al contents examined. The role of alloy scattering and interface roughness scattering is also discussed.
    Applied Physics Letters 01/2010; 97. · 3.52 Impact Factor
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    ABSTRACT: We report on the intrinsically limited low-field mobility of the two-dimensional electron gas (2DEG) in gated AlGaN/GaN and AlGaN/AlN/GaN heterostructures. Monte Carlo transport simulations are carried out to calculate the room-temperature 2DEG mobilities in dependence on the electron sheet density. The simulated 2DEG mobilities are compared to the phonon-limited mobility of bulk GaN. We estimate a maximum 2DEG mobility of about 2700 cm <sup>2</sup>  V <sup>-1</sup>  s <sup>-1</sup> for an electron sheet density of ∼5×10<sup>12</sup> cm <sup>-2</sup> , which remarkably exceeds the phonon-limited bulk mobility of 1520 cm <sup>2</sup>  V <sup>-1</sup>  s <sup>-1</sup> . By reducing the electron sheet density below 5×10<sup>12</sup> cm <sup>-2</sup> , i.e., in a weak electron quantum confinement regime, the room-temperature 2DEG mobility gradually decreases and approaches the phonon-limited bulk value for vanishing quantum confinement. The insertion of a thin AlN barrier interlayer improves transport properties of the 2DEG and the mobility substantially increases due to a suppression of the alloy scattering.
    Journal of Applied Physics 08/2009; · 2.19 Impact Factor
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    ABSTRACT: We report on the electron transport in zinc blende InN simulated by the ensemble Monte Carlo method. To obtain the relevant band structure parameters, ab initio calculations have been carried out. Then, Monte Carlo transport simulations at room temperature and over a wide range of carrier concentrations have been performed. We obtain a steady-state peak drift velocity around 3.3×10<sup>7</sup> cm / s at an electric field of 55 kV/cm. For low-doped material, a room-temperature low-field mobility of about 6000 cm <sup>2</sup>/ V   s is calculated. A comparison with wurtzite InN does not reveal an advantage for the zinc blende InN phase regarding the electron transport.
    Applied Physics Letters 02/2009; · 3.52 Impact Factor
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    ABSTRACT: AlGaN/GaN pH sensitive devices were functionalized and passivated for the use as selective bio-sensors. For the passivation, a multilayer of SiO2 and SiNx is proposed, which stabilizes the pH-sensor, is biocompatible and has no negative impact on the following bio-functionalization. The functionalization of the GaN-surface was achieved by covalent bonding of 10-amino-dec-1-ene molecules by a photochemical process. After two different surface preparations islands of TFAAD are growing on the sensor surface by exposure with UV-light. In dependence on the surface pre-treatment and the illumination wavelength the first monolayer is completed after 3 h or 7 h exposure time dependent on the pre-treatment and illumination wavelength. Further exposure results in thicker films as a consequence of cross polymerization. The bonding to the sensor surface was analyzed by X-ray photoelectron spectroscopy, while the thickness of the functionalization was determined by atomic force microscopy scratching experiments. These functionalized devices based on the pH-sensitive AlGaN/GaN ISFET will establish a new family of adaptive, selective biomolecular sensors such as selective, reusable DNA sensors.
    MRS Online Proceeding Library 12/2008; 1202.
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    ABSTRACT: The structural, chemical and electron transport properties of In2O3/InN heterostructures and oxidized InN epilayers are reported. It is shown that the accumulation of electrons at the InN surface can be manipulated by the formation of a thin surface oxide layer. The epitaxial In2O3/InN heterojunctions show an increase in the electron concentration due to the increasing band banding at the heterointerface. The oxidation of InN results in improved transport properties and in a reduction of the sheet carrier concentration of the InN epilayer very likely caused by a passivation of surface donors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2008; 5(2):495 - 498.
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    ABSTRACT: We report on the compositional dependencies of electron transport and photoconductive properties for ultrathin metal-semiconductor-metal photodetectors based on In-rich InxGa1-xN alloys. For a In0.64Ga0.36N/GaN structure, the rise time close to the RC constant at low fields has been measured along with a transparency of ~77% and an absorbance of ~0.2 at a wavelength of 632 nm. The electron density profiles and low-field mobilities for different compositions of InGaN have been calculated by numerically solving the Schrödinger and Poisson equations and applying the ensemble Monte Carlo method, respectively. It was demonstrated that in ultrathin InxGa1-xN/GaN (0.5
    Journal of Applied Physics 01/2008; 103. · 2.19 Impact Factor
  • R. Granzner, F. Schwierz, V.M. Polyakov
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    ABSTRACT: An analytical model describing the effects of 2-D quantum-mechanical carrier confinement on the threshold voltage V<sub>th</sub> of multiple-gate MOSFETs with rectangular cross section is developed. The model is verified by a comparison with self-consistent solutions of 1-D and 2-D Schroumldinger and Poisson equations. It is shown that: 1) the model results asymptotically approach the case of 1-D confinement in single-gate silicon-on-insulator or double-gate MOSFETs if one body dimension becomes larger than 20 nm and 2) the effect of 2-D confinement is remarkably stronger than a simple combination of two 1-D quantization effects.
    IEEE Transactions on Electron Devices 10/2007; · 2.36 Impact Factor
  • V M Polyakov, F Schwierz
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    ABSTRACT: We investigate the band structure effects on the terahertz (THz) electric field emission from unbiased InN and InAs surfaces using the ensemble Monte Carlo (MC) simulation method. It is shown that the nonparabolicity of the central conduction band valley and the energy separations between the central and satellite valleys are crucial parameters to provide a high efficiency of THz surface emitters. The THz emission from InN and InAs surfaces exposed to femtosecond laser pulses are compared over a wide range of photon energies from 0.85 eV (λ = 1550 nm) up to 1.55 eV (λ = 800 nm). We also demonstrate that the relatively high intrinsic carrier concentration of InAs (~8 × 1014 cm−3) has a negligibly small effect on the THz surface emission.
    Semiconductor Science and Technology 08/2007; 22(9):1016. · 2.21 Impact Factor

Publication Stats

512 Citations
89.95 Total Impact Points


  • 2010–2012
    • Fraunhofer Institute for Applied Solid State Physics IAF
      Freiburg, Baden-Württemberg, Germany
  • 1997–2010
    • Technische Universität Ilmenau
      • • Institute of Physics
      • • Institut für Mikro- und Nanotechnologien
      Ilmenau, Thuringia, Germany
  • 1994–1996
    • Universität Kassel
      • Center for Interdisciplinary Nanostructure Science and Technology (CINSaT)
      Cassel, Hesse, Germany