V. M. Polyakov

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

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Publications (54)93.32 Total impact

  • F. Benkhelifa · S. Muller · V. M. Polyakov · S. Breuer · H. Czap · C. Manz · M. Mikulla · O. Ambacher ·

    ECS Transactions 10/2015; 69(11):65-70. DOI:10.1149/06911.0065ecst
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    ABSTRACT: Large area terahertz emitters based on the lateral photo-Dember effect in InN (indium nitride) are presented. The formation of lateral photo-Dember currents is induced by laser-illumination through a microstructured metal cover processed onto the InN substrate, causing an asymmetry in the lateral photogenerated charge carrier distribution. Our design uses simple metal structures, which are produced by conventional two-dimensional micro-structuring techniques. Having favoring properties as a photo-Dember material InN is particularly well-suited as a substrate for our emitters. We demonstrate that the emission intensity of the emitters can be significantly influenced by the structure of the metal cover leaving room for improvement by optimizing the masking structures.
    Applied Physics Letters 09/2015; 107(11):111102. DOI:10.1063/1.4930233 · 3.30 Impact Factor
  • Fouad Benkhelifa · Stefan Muller · V.M. Polyakov · Oliver Ambacher ·
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    ABSTRACT: In this letter, we report on polarization charge engineering enabling normally-off operation for a double-heterostructure Al0.26Ga0.74N/GaN/Al0.07Ga0.93N-based field effect transistors (DHFETs) using a 35-nm-thick undoped GaN layer underneath the gate metallization. The combined effect of the negative polarization charge induced by the AlGaN back barrier and the undoped GaN gate layer ensures the total depletion of the channel, and provides a positive threshold voltage. The fabricated DHFET exhibits normally-off operation with a threshold voltage of 1.2 V, a maximum drain current density of 370 mA/mm, and a high ON/OFF current ratio of $10^{7}$ , at a gate bias of 7 V. A transistor with gate–drain distance of 6 $mu text{m}$ demonstrates 300 V off-state breakdown voltage.
    IEEE Electron Device Letters 09/2015; 36(9):905-907. DOI:10.1109/LED.2015.2459597 · 2.75 Impact Factor
  • R. Granzner · V.M. Polyakov · V. Cimalla · O. Ambacher · F. Schwierz ·
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    ABSTRACT: The design and performance of bias-free InN-based THz emitters that exploit lateral photocurrents is studied by means of numerical simulations. We use a drift-diffusion model with adjusted carrier temperatures and mobilities. The applicability of this approach is demonstrated by a comparison with results from Monte-Carlo simulations. We consider a simple but robust lateral emitter concept using metal stripes with two different thicknesses with one of them being thin enough to be transparent for THz radiation. This arrangement can be easily multiplexed and the efficiency of this concept has already been demonstrated by experiment for GaAs substrates. In the present study, we consider InN, which is known to be an efficient photo-Dember emitter because of its superior transport properties. Our main focus is on the impact of the emitter design on the emission efficiency assuming different operation principles. Both the lateral photo-Dember (LPD) effect and built-in lateral field effects are considered. The appropriate choice of the metal stripe and window geometry as well as the impact of surface Fermi level pinning are investigated in detail, and design guidelines for efficient large area emitters using multiplexed structures are provided. We find that InN LPD emitters do not suffer from Fermi level pinning at the InN surface. The optimum emission efficiency is found for LPD emitter structures having 200 nm wide illumination windows and mask stripes. Emitter structures in which lateral electric fields are induced by the metal mask contacts can have a considerably higher efficiency than pure LPD emitters. In the best case, the THz emission of such structures is increased by one order of magnitude. Their optimum window size is 1 μm without the necessity of a partially transparent set of mask stripes.
    Journal of Applied Physics 07/2015; 118(4). DOI:10.1063/1.4927267 · 2.18 Impact Factor
  • Ralf Granzner · Vladimir M. Polyakov · Christian Schippel · Frank Schwierz ·
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    ABSTRACT: An empirical model for the effective electron mobility in silicon nanowires (SiNWs) is presented. The model is based on published mobility data from numerical simulations of electron transport in SiNWs with different cross sections. Both phonon scattering and surface roughness scattering as well as the impact of the effective vertical field are considered. A comparison with a variety of experimental mobility data from the literature shows that the model can be treated as a reference for benchmarking different NW technologies. The effective field dependence is modeled by a simple expression making our mobility model very efficient for the use in numerical device simulators or in analytical MOSFET models.
    IEEE Transactions on Electron Devices 11/2014; 61(11):3601-3607. DOI:10.1109/TED.2014.2354254 · 2.47 Impact Factor
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    ABSTRACT: The lifetime and stability of AlGaN/GaN heterostructure field effect transistors at high power levels can be enhanced by introducing field plates to reduce electric field peaks in the gate-drain region. Simulations of the electric field distribution along the channel using the 2D ATLAS software from Silvaco indicate that above a characteristic drain source voltage three spatially separated electric field peaks appear, one located at the drain-side edge of the gate foot, one at the end of the drain-sided gate field plate, and one at the end of the source shield field plate. The close correlation between lateral electric field and the electroluminescence due to hot electron related intra-band transitions can be very helpful when optimizing the electric field distribution in high power devices. Electroluminescence microscopy images of devices with gate and source shield field plate reveal the peaks located at the locations of enhanced electric field. By studying the voltage dependence of the electroluminescence peaks the influence of the field plates on the electric field distribution in source drain direction can be visualized.
    Acta Physica Polonica Series a 03/2014; 125(4):982-985. DOI:10.12693/APhysPolA.125.982 · 0.53 Impact Factor
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    ABSTRACT: In this work we report on non-resonant piezoelectric micro-energy-harvesters based on corrugated membranes made from aluminium nitride. These harvesters were designed to respond efficiently to aperiodic mechanical impact at low repetition rates. The piezogeneration efficiency of non-resonant systems was compared to conventional resonant devices like tensile stressed membranes and cantilevers. Special attention was paid to system properties that allow an operation at low frequencies in viscous media with high damping. Classical analytical modelling was employed to describe the corrugated microsystems with the aim to calculate the feasible energy outcome and to optimise electro-mechanical coupling.
    Sensors and Actuators A Physical 06/2013; 195:32–37. DOI:10.1016/j.sna.2013.02.001 · 1.90 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. DOI:10.1088/0268-1242/27/12/125003 · 2.19 Impact Factor
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    ABSTRACT: We investigated the impact of SF6 plasma treatments on the electronic transport properties of GaN/AlGaN/GaN heterostructures by employing different plasma conditions as well as annealing in nitrogen atmosphere at 425 °C. The electrical properties are characterized by Hall-effect measurements while electron spectroscopy and X-ray measurements are used to investigate changes in the surface chemical composition and in the layer structure, respectively. It is demonstrated that plasma treatments strongly affect the 2DEG properties of the heterostructure due to altering of the surface potential accompanied by the formation of a thin fluorinated amorphous film. Increasing the DC bias voltage for the plasma treatment leads to an additional degradation of the mobility caused by incorporation of fluorine into the heterostructure interface. Furthermore, the thin GaN cap layer is etched by plasma treatments with higher bias potential, which increases the carrier density at the interface (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2012; 9(3-4):938-941. DOI:10.1002/pssc.201100418
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    ABSTRACT: We investigated the impact of fluorine and nitrogen plasma treatments on the electronic transport properties of GaN/AlGaN/GaN heterostructures by employing different plasma processes and subsequent annealing in nitrogen atmosphere at 425 °C. It is demonstrated that the plasma treatments affect the 2-dimensional electron gas (2DEG) properties of the HEMT structure resulting in a decrease of the surface potential. In order to understand the physical mechanisms we have undertaken an extensive study of the influence of plasma processing on the 2DEG properties. The electrical properties of the 2DEG are characterized by Hall measurements while X-ray photoelectron spectroscopy (XPS) is used to investigate changes in surface chemical composition. Changes of the DC bias voltage have a strong effect on the mobility presumably by incorporation of fluorine and nitrogen close to the channel. To confirm this assumption, Monte Carlo simulations were carried out, taking into account surface potential and negatively charged fluorine acceptors acting as additional scattering centers within the 2DEG (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2012; 9(3-4):1096-1098. DOI:10.1002/pssc.201100210
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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). DOI:10.1063/1.3688219 · 2.18 Impact Factor
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    V. M. Polyakov · V. Cimalla · V. Lebedev · K. Köhler · S. Müller · P. Waltereit · O. Ambacher ·
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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 10/2010; 97(14). DOI:10.1063/1.3499656 · 3.30 Impact Factor
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    ABSTRACT: Results from on-wafer electroluminescence (EL) microscopy on AlGaN/GaN high-electron-mobility transistors with leakage currents varying over four orders of magnitude are presented. In the off-state region the integrated EL intensity is proportional to the leakage current and independent of gate width for the devices under study. The slope of the integrated EL–leakage current dependence is determined by the electrical field in the source–drain direction. The influence of the GaN cap thickness is small or even negligible for higher drain bias. Stress during accelerated aging results in enhanced degradation for areas of enhanced leakage current and/or electric field values. KeywordsElectroluminescence microscopy-AlGaN/GaN HEMT-leakage current
    Journal of Electronic Materials 06/2010; 39(6):756-760. DOI:10.1007/s11664-010-1120-9 · 1.80 Impact Factor
  • 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) 06/2010; 207(6). DOI:10.1002/pssa.200983539 · 1.21 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) 02/2010; 7(2):173 - 176. DOI:10.1002/pssc.200982494
  • Ralf Granzner · Mario Kittler · Frank Schwierz · V. M. Polyakov ·
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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.
    01/2010; DOI:10.1109/ESSDERC.2010.5618189
  • V. M. Polyakov · F. Schwierz · I. Cimalla · M. Kittler · B. Lubbers · A. Schober ·
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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; 106(2-106):023715 - 023715-5. DOI:10.1063/1.3174441 · 2.18 Impact Factor
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    V. M. Polyakov · F. Schwierz · F. Fuchs · J. Furthmuller · F. Bechstedt ·
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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; 94(2-94):022102 - 022102-3. DOI:10.1063/1.3059570 · 3.30 Impact Factor
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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 04/2008; 103(7). DOI:10.1063/1.2903146 · 2.18 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; 54(9-54):2562 - 2565. DOI:10.1109/TED.2007.902167 · 2.47 Impact Factor

Publication Stats

657 Citations
93.32 Total Impact Points


  • 2010-2015
    • 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