A. Waag

Technische Universität Braunschweig, Brunswyck, Lower Saxony, Germany

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Publications (416)768.21 Total impact

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    ABSTRACT: The performance of a low-cost partially integrated cantilever-based airborne nanoparticle (NP) detector (CANTOR-1) is evaluated in terms of its real-time measurement and robustness. The device is used for direct reading of exposure to airborne carbon engineered nanoparticles (ENPs) in indoor workplaces. As the main components, a miniaturized electrostatic aerosol sampler and a piezoresistive resonant silicon cantilever mass sensor are employed to collect the ENPs from the air stream to the cantilever surfaces and to measure their mass concentration, respectively. Moreover, to realize a real-time measurement, a frequency tracking system based on a phase-locked loop (PLL) is built and integrated into the device. Long-term ENP exposure and a wet ultrasonic cleaning method are demonstrated to estimate the limitation and extend the operating lifetime of the developed device, respectively. By means of the device calibrations performed with a standard ENP monitoring instrument of a fast mobility particle sizer (FMPS, TSI 3091), a measurement precision of ENP mass concentrations of < 55% and a limit of detection (LOD) of < 25 μg m−3 are obtained.
    03/2015; 4(1):111-123. DOI:10.5194/jsss-4-111-2015
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    ABSTRACT: This paper presents the design, fabrication, and use of silicon nanowire (SiNW) arrays-patterned microcantilever sensors excited in the in-plane resonance mode to enhance the detection of airborne particulate matter (PM). Electrothermal excitation elements of p-diffused heating resistors were introduced in the current sensor system to replace the formerly used external piezoceramic stack actuator. The sensors exhibited high measured quality factors (Q-factors) of 4702 ± 102 during their in-plane mode operations in air, which are four times larger than those of the fundamental out-of-plane mode. To selectively define arrays of vertical SiNWs on the surface of the micromechanical cantilever, nanoimprint lithography (NIL) is combined with conventional photolithography. The diameter and position of the SiNWs can be adjusted depending on the nanoimprint stamp with the smallest cylindrical pattern possible down to 50 nm in diameter. By modifying the resonator surface, the PM sampling efficiency can be improved by a factor of 1.5 greater than that of a corresponding plain cantilever in a cigarette smoke exposure experiment because of the rise in collection surface area of the sensor given by the SiNWs.
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    ABSTRACT: Spin coherence of resident electrons and holes is measured in ZnSe-based quantum wells by means of time-resolved Kerr rotation technique. At a temperature of 1.8 K spin dephasing time for localized electrons can be as long as 33 ns, and for holes of 0.8 ns. Electron spin precession is clearly observed in a wide temperature range up to 230 K. The electron spin dephasing becomes much shorter of 0.2 ns for the quantum well with a high-density electron gas. Using vector magnet all components of the g-factors tensor are evaluated for the electrons and heavy-holes, revealing strong anisotropy for the heavy-holes.
    physica status solidi (b) 09/2014; 251(9). DOI:10.1002/pssb.201350233 · 1.49 Impact Factor
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    ABSTRACT: The efficiency of the Mn-spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type-II band alignment by means of time-resolved photoluminescence and pump-probe reflectivity. An essential role in the heating is played by multiple spin-flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system.
    physica status solidi (b) 09/2014; 251:1694. DOI:10.1002/pssb.201350320 · 1.49 Impact Factor
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    ABSTRACT: The potential use of nanoelectromechanical systems (NEMS) created in silicon nanopillars (SiNPLs) is investigated in this work as a new generation of aerosol nanoparticle (NP)-detecting device. The sensor structures are created and simulated using a finite element modeling (FEM) tool of COMSOL Multiphysics 4.3b to study the resonant characteristics and the sensitivity of the SiNPL for femtogram NP mass detection in 3-D structures. The SiNPL arrays use a piezoelectric stack for resonance excitation. To achieve an optimal structure and to investigate the etching effect on the fabricated resonators, SiNPLs with different designs of meshes, sidewall profiles, heights, and diameters are simulated and analyzed. To validate the FEM results, fabricated SiNPLs with a high aspect ratio of approximately 60 are used and characterized in resonant frequency measurements where their results agree well with those simulated by FEM. Furthermore, the deflection of a SiNPL can be enhanced by increasing the applied piezoactuator voltage. By depositing different NPs [i.e., gold (Au), silver (Ag), titanium dioxide (TiO2), silicon dioxide (SiO2), and carbon black NPs] on the SiNPLs, the decrease of the resonant frequency is clearly shown confirming their potential to be used as airborne NP mass sensor with femtogram resolution level. A coupling concept of the SiNPL arrays with piezoresistive cantilever resonator in terms of the mass loading effect is also studied concerning the possibility of obtaining electrical readout signal from the resonant sensors.
    Microsystem Technologies 04/2014; 20(4-5). DOI:10.1007/s00542-013-1992-8 · 0.88 Impact Factor
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    ABSTRACT: Silicon resonant sensors with large surface area-to-volume ratios provide high weighing sensitivity. This fact implies the possibility for detection of slight mass changes [i.e. by attached nanoparticles (NPs)]. Vertical silicon nanowire (SiNW) resonators are therefore suitable for exposure assessment or airborne NPs. SiNW arrays are top-down fabricated by nanolithography and subsequent inductively coupled plasma reactive ion etching at cryogenic temperature. Nanolithography is performed by conventional UV-lithography and nanoimprint for even smaller structures. Wire diameters are further reduced by multiple thermal oxidations and oxide stripping at times. Parameter effects of cryogenic dry etching are studied for SiNW arrays.
    Microsystem Technologies 04/2014; 20(4). DOI:10.1007/s00542-013-2032-4 · 0.88 Impact Factor
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    ABSTRACT: In n-type GaN, an upward band bending of about 1 eV is caused by negative charge at the surface. UV light reduces the band bending by creating a surface photovoltage (SPV), which can be measured by a Kelvin probe. Previously, we reported a fast SPV signal of about 0.6 eV in undoped and moderately doped GaN. In this work, we have studied degenerate GaN co-doped with Zn and Si, with a Si concentration of about 10(19) cm(-3) and a Zn concentration of 6x10(17) cm(-3). At room temperature, a fast component of about 0.6 eV was observed. However, after preheating the sample at 600 K for one hour and subsequently cooling the sample to 300 K (all steps performed in vacuum), the fast component disappeared. Instead, a very slow (minutes) and logarithmic in time rise of the SPV was observed with UV illumination. The total change in SPV was about 0.4 eV. This slow SPV transient can be reversibly converted into the "normal" fast (subsecond) rise by letting air or dry oxygen in at room temperature. Possible explanations of the observed unusual SPV transients are discussed.
    27th International Conference on Defects in Semiconductors (ICDS); 02/2014
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    ABSTRACT: Transmission and scanning electron microscopy have been used to characterise GaN/InGaN 3D nanostructures grown on patterned GaN/sapphire substrates by metal organic vapour phase epitaxy (MOVPE). It has been found that the growth of well ordered arrays of such nanostructures, containing multiple quantum wells on non-polar side-facets, can be achieved with a low density of defects. Growth changes and surface morphology play a major role in the nucleation of any defects present. The nanostructure morphology has been investigated and differing growth rates on adjacent facets studied. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 02/2014; 11(3-4). DOI:10.1002/pssc.201300522
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    ABSTRACT: We investigate the thermal equilibrium dynamics of electron spins bound to donors in nanoporous ZnO by optical spin noise spectroscopy. The spin noise spectra reveal two noise contributions: A weak spin noise signal from undisturbed localized donor electrons with a dephasing time of 24 ns due to hyperfine interaction and a strong spin noise signal with a spin dephasing time of 5 ns which we attribute to localized donor electrons which interact with lattice defects.
    31st International Conference on the Physics of Semiconductors (ICPS); 12/2013
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    ABSTRACT: Columnar structures of III-V semiconductors recently attract considerable attention because of their potential applications in novel optoelectronic and electronic devices. In the present study, the mechanisms for the growth of catalyst-free self-organized GaN columns on sapphire substrate by metal organic vapor phase epitaxy have been thoroughly investigated. The growth behaviours are strongly affected by the choice of carrier gas. If pure nitrogen is used, Ga droplets are able to accumulate on the top of columns during growth, and they are converted into a high quality GaN layer during the cool down phase due to nitridation. Hydrogen as the carrier gas can improve the optical quality of the overall GaN columns substantially, and in addition increase the vertical growth rate. In this case, no indication of Ga droplets could be detected. Furthermore, silane doping during the growth promotes the vertical growth in both cases either pure nitrogen or pure hydrogen as the carrier gas.
    Journal of Crystal Growth 12/2013; 384:61. DOI:10.1016/j.jcrysgro.2013.09.015 · 1.70 Impact Factor
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    ABSTRACT: In the past few years, tremendous progress has been demonstrated on epitaxial growth and processing of group III nitride nano- and microrods (NAMs). This has also enabled the fabrication of optoelectronic devices based on NAMs as active elements. However, their efficiency is still far behind the performance of conventional GaN-based light emitting diodes (LEDs). This Review presents the most recent activities on the growth and processing of NAMs exhibiting a core-shell geometry, i.e. structures which consist of an active region shell layer wrapped around a three-dimensional (3D) core, which allow for an enormous increase in active area compared to planar technology. The most common growth approaches using metalorganic vapour phase epitaxy are described and evaluated with particular regard to their potential for solid state lighting applications. Examples for the unique properties of 3D NAMs are presented including their excellent crystalline quality. Furthermore, factors limiting the overall performance of 3D core-shell LEDs are revealed and the potential of overcoming these limitations are discussed. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (RRL) - Rapid Research Letters 10/2013; 7(10):800-814. DOI:10.1002/pssr.201307250 · 2.14 Impact Factor
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    ABSTRACT: Arrays of dislocation free uniform Ga-polar GaN columns have been realized on patterned SiOx/GaN/sapphire templates by metal organic vapor phase epitaxy using a continuous growth mode. The key parameters and the physical principles of growth of Ga-polar GaN three-dimensional columns are identified, and their potential for manipulating the growth process is discussed. High aspect ratio columns have been achieved using silane during the growth, leading to n-type columns. The vertical growth rate increases with increasing silane flow. In a core-shell columnar LED structure, the shells of InGaN/GaN multi quantum wells and p-GaN have been realized on a core of n-doped GaN column. Cathodoluminescence gives insight into the inner structure of these core-shell LED structures.
    Crystal Growth & Design 08/2013; 13:3475. DOI:10.1021/cg4003737 · 4.89 Impact Factor
  • Andrej Stranz · Andreas Waag · Erwin Peiner
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    ABSTRACT: Deep reactive-ion etching at cryogenic temperatures (cryo-DRIE) has been used to produce arrays of silicon nanowires (NWs) for thermoelectric (TE) power generation devices. Using cryo-DRIE, we were able to fabricate NWs of large aspect ratios (up to 32) using a photoresist mask. Roughening of the NW sidewalls occurred, which has been recognized as beneficial for low thermal conductivity. Generated NWs, which were 7 μm in length and 220 nm to 270 nm in diameter, were robust enough to be stacked with a bulk silicon chip as a common top contact to the NWs. Mechanical support of the NW array, which can be created by filling the free space between the NWs using silicon oxide or polyimide, was not required. The Seebeck voltage, measured across multiple stacks of up to 16 bulk silicon dies, revealed negligible thermal interface resistance. With stacked silicon NWs, we observed Seebeck voltages that were an order of magnitude higher than those observed for bulk silicon. Degradation of the TE performance of silicon NWs was not observed for temperatures up to 470°C and temperature gradients up to 170 K.
    Journal of Electronic Materials 07/2013; 42(7). DOI:10.1007/s11664-013-2590-3 · 1.80 Impact Factor
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    ABSTRACT: The huge and intelligent processing power of three-dimensional (3D) biological “processors” like the human brain with clock speeds of only 0.1 kHz is an extremely fascinating property, which is based on a massively parallel interconnect strategy. Artificial silicon microprocessors are 7 orders of magnitude faster. Nevertheless, they do not show any indication of intelligent processing power, mostly due to their very limited interconnectivity. Massively parallel interconnectivity can only be realized in three dimensions. Three-dimensional artificial processors would therefore be at the root of fabricating artificially intelligent systems. A first step in this direction would be the self-assembly of silicon based building blocks into 3D structures. We report on the self-assembly of such building blocks by molecular recognition, and on the electrical characterization of the formed assemblies.
    Langmuir 06/2013; 29(26). DOI:10.1021/la401558f · 4.46 Impact Factor
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    ABSTRACT: The growth mechanism of catalyst-free self-organized GaN nuclei and three-dimensional columns on sapphire by metal organic vapour phase epitaxy (MOVPE) is investigated. Temperature- and time-dependent growth is performed. The growth behaviour can be characterized by two different kinetic regimes: mass-transport-limited growth and thermodynamically limited growth. The sum of activation energies for thermodynamic barrier of nucleation and for surface diffusion/mass-transport limitation, i.e. Whet +Ed, is 0.57 eV in the 'low'-temperature region and 2.43 eV in the 'high'-temperature region. GaN columns grown under the same conditions have very comparable height, which is not dependent on their diameter or the distance to other columns. Therefore, the growth rate is presumably limited by the incorporation rate on the top surface of columns. The height and diameter at the top of the GaN columns increase linearly with time and no height limit is observed. The GaN columns can reach more than 40 µm in height. Moreover, the investigated GaN columns are Ga-polar.
    Journal of Physics D Applied Physics 05/2013; 46(20):205101. DOI:10.1088/0022-3727/46/20/205101 · 2.72 Impact Factor
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    ABSTRACT: A silicon resonant cantilever sensor is developed for detection of airborne nanoparticles (NPs) by monitoring the change in resonant frequency induced by an additional mass of trapped NPs. A piezoelectric stack actuator and a piezoresistive strain gauge are involved in the sensor system in order to actuate and detect the oscillation of cantilever sensor, respectively. An electrostatic precipitator is employed to trap the NPs on the cantilever surface. The proposed sensor reveals a mass sensitivity of 10 Hz/ng and a quality factor of 1206 while operated in the fundamental flexural mode. As necessary for an application under workplace conditions the limitations of the sensor sensitivity imposed by the environment are investigated, i.e., the influences of temperature, relative humidity, and pressure on the sensor are measured.
    Sensors and Actuators B Chemical 04/2013; 180:77–89. DOI:10.1016/j.snb.2012.04.003 · 4.10 Impact Factor
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    ABSTRACT: Gallium nitride samples codoped with zinc and silicon were fabricated by metal organic chemical vapor deposition. Optical properties of these samples with lowlevel doped acceptor are investigated by temperature dependent photoluminescence (PL) measurements. PL spectrum shows three peaks labeled as near band edge (NBE), blue luminescence (BL), and yellow luminescence (YL).BL originates from ZnGa acceptors in GaN:Zn,Si. It is quenched above 250 K for all samples measured at excitation density of Pexc = 0.2 Wcm−2. The concentration of acceptors responsible for the BL band is estimated for relatively low Zn concentrations by comparing integrated PL intensity of NBE and Zn-related BL bands at 200 K (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2013; 10(3). DOI:10.1002/pssc.201200669
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    ABSTRACT: Photoluminescence (PL) from high-quality GaN codoped with silicon and zinc was investigated in detail. The internal quantum efficiency (IQE) of PL was determined from the analysis of the dependencies of the PL intensity on the excitation intensity and temperature, and the simulation of these dependencies with a phenomenological model based on rate equations. The model reproduces an important phenomenon: the quenching of a recombination channel with a high IQE causes a rise in efficiency of all the other PL bands. Quantitative analysis of this phenomenon allows one to determine reliably the absolute IQE of PL. The absolute IQE of the PL in GaN co-doped with Si and Zn exceeds 90%, with the largest contribution coming from the blue luminescence band associated with the ZnGa acceptor. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2013; 10(3):507-510. DOI:10.1002/pssc.201200664
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    ABSTRACT: Selective area growth of GaN rods by metalorganic vapor phase epitaxy has attracted great interest due to its novel applications in optoelectronic and photonics. In this work, we will present the dependence of GaN rod morphology on various growth parameters i.e. growth temperature, H2/N2 carrier gas concentration, V/III ratio, total carrier gas flow and reactor pressure. It is found that higher growth temperature helps to increase the aspect ratio of the rods, but reduces the height homogeneity. Furthermore, H2/N2 carrier gas concentration is found to be a critical factor to obtain vertical rod growth. Pure nitrogen carrier gas leads to irregular growth of GaN structure, while an increase of hydrogen carrier gas results in vertical GaN rod growth. Higher hydrogen carrier gas concentration also reduces the diameter and enhances the aspect of the GaN rods. Besides, increase of V/III ratio causes reduction of the aspect ratio of N-polar GaN rods, which could be explained by the relatively lower growth rate on (000-1) N-polar top surface when supplying more ammonia. In addition, an increase of the total carrier gas flow leads to a decrease in the diameter and the average volume of GaN rods. These phenomena are tentatively explained by the change of partial pressure of the source materials and boundary layer thickness in the reactor. Finally, it is shown that the average volume of the N-polar GaN rods keeps a similar value for a reactor pressure PR of 66 and 125 mbar, while an incomplete filling of the pattern opening is observed with PR of 250 mbar. Room temperature photoluminescence spectrum of the rods is also briefly discussed.
    Journal of Crystal Growth 02/2013; 364:149–154. DOI:10.1016/j.jcrysgro.2012.11.027 · 1.70 Impact Factor

Publication Stats

6k Citations
768.21 Total Impact Points


  • 2004–2014
    • Technische Universität Braunschweig
      • • Institut für Halbleitertechnik
      • • Institut of Applied Physics
      Brunswyck, Lower Saxony, Germany
  • 1970–2011
    • Universität Ulm
      Ulm, Baden-Württemberg, Germany
  • 2009
    • Universität Paderborn
      • Department of Physics
      Paderborn, North Rhine-Westphalia, Germany
  • 2007
    • Purdue University
      • School of Electrical and Computer Engineering
      West Lafayette, IN, United States
  • 1989–2004
    • University of Wuerzburg
      • • Division of Experimental Physics VII
      • • Institute of Physics
      Würzburg, Bavaria, Germany
  • 1999
    • Ioffe Physical Technical Institute
      Sankt-Peterburg, St.-Petersburg, Russia
  • 1998–1999
    • Russian Academy of Sciences
      Moskva, Moscow, Russia
    • Radboud University Nijmegen
      • High Field Magnet Laboratory (HFML)
      Nymegen, Gelderland, Netherlands
  • 1994
    • Philipps University of Marburg
      Marburg, Hesse, Germany