Topic 1. Chemical and Biochemical Sensors
Opto-chemical sensor system based on InGaN/GaN nanowires for detection of oxidizing gases 1
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Sumit Paul , Andreas Helwig , Gerhard Müller , Florian Furtmayr , Jörg Teubert and Martin Eickhoff 1 EADS Innovation Works, 81663 Munich, Germany 2 I. Physikalisches Institut, Justus-Liebig-Universität Gießen, 35392 Gießen, Germany 3 Walter Schottky Institute, Technical University of Munich, 85748 Garching, Germany Corresponding e-mail address:
[email protected]
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Summary We report on an all-optical sensor concept that employs GaN/InGaN nanowire heterostructures (NWHs) as opto-chemical transducers. The NWHs were grown by plasmaassisted molecular beam epitaxy on silicon and optically transparent sapphire substrates. These NWHs exhibit an efficient photoluminescence which is quenched when the NWHs are exposed to oxidizing gases such as O2, NO2 and O3. These quenching effects are strongest at room temperature and decrease as the transducer temperature is raised. At room temperature minimum detectable gas concentrations are 1000ppm (O2), 50ppb (NO2), and 10ppb (O3). The proposed opto-chemical transducer principle is best suited for safety-critical applications where a reliable media separation between the medium to be sensed and the sensor’s electrical circuitry is required.
Sensing principle Group III-N nanostructures yield an efficient PL at and above RT, which can be influenced by external electric fields [1]. This electrical field dependence can be transformed into a chemical sensitivity when exposed to a suitable medium. The PL quenching of GaN/AlGaN NWHs at room temperature and above was interpreted to be due to an increased nonradiative recombination of photoexcited carriers at surface states in the presence of oxidizing gases.
Experimental The GaN/InGaN NWHs consist of a GaN nanowire base part with a length of approx. 400 nm, followed directly by a number of GaN/InGaN heterostructures. The multi-quantum well structures consist of a series of InGaN nanodisks separated by 7nm thick GaN barrier layers and a 20 nm GaN cap layer at the top surface. Fig.1a shows a schematic drawing of the overall sample structure, a real-space picture of the NWs is shown in Fig.1b. In our experiments the thickness d of the nanodisks was varied within the range 1.3nm < d < 4 nm. Typical wire diameters ranged between 25 nm up to 50 nm. In order to achieve light emission in the desired wavelength range, appropriate choices of the In molar fraction x were made as the InxGa1-xN nanodisks were deposited. A schematic drawing of the all-optical sensor system setup is displayed in Fig.2. The excitation energy is provided via commercial UV power LEDs emitting at 365 nm. A photomultiplier tube (PMT) is used as detection device.
Results Fig.3a presents the response of one of our opto-chemical transducers to increasingly higher O2 concentrations. The photomultiplier output voltage is plotted as signal. The sequence of gas pulses supplied to this transducer consisted of five pulses of O2 with the O2 concentration increasing from 0.2 - 2.1% O2/N2, separated by periods of pure N2 exposure. This first set of data clearly reveals that increasing O2 concentrations cause decreasing PL output signal levels. Due to the much stronger oxidising behaviour of NO2, much lower gas concentrations were needed to observe sizeable PL intensity changes. This fact is illustrated in Fig.3b.
GaN cap GaN barrier (7 nm) InGaN ND (1.3 - 4 nm)
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GaN base
Si interlayer Sapphire substrate
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Fig.1: (a) Sketch of the nanorod pillars with interspersed InGaN nanodisks. The total length of the nanorods is approx. 400 nm. The thickness of the nanodisks ranges between 1.3nm and 4nm; (b) cross-sectional CTEM image of a transducer sample consisting of this same type of nanorods. Illumination fiber
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Gas outlet Illumination fiber LED -Power LED 365nm
f exFilter for excitation side, bandpass 357nm OD 6 f em Filter for emission side, longpass 409nm OD 6
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2.70 0.2% O2
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2.64 Carrier gas: Nitrogen 2.62 Room temperature
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Fig.2: Schematics of the optical setup featuring three light fibres and an opto-chemical GaN/InGaN transducer (vertical red line).
Carrier gas:Synthetic air+30% humidity Room temperature
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Fig.3: a) PL output intensity in response to increasing concentrations of O2 in N2; b) Response of the PL intensity to a sequence of NO2 concentration steps. The carrier gas was humidified synthetic air with a relative humidity of 30%. All measurements were performed at room temperature. References [1] O. Weidemann, P. K. Kandaswamy, E. Monroy, G. Jegert, M. Stutzmann, and M. Eickhoff, “GaN quantum dots as optical transducers for chemical sensors” Applied Physics Letters, AIP, 2009, 94, 113108. [2] J. Teubert, P. Becker, F. Furtmayr, M. Eickhoff (2011): GaN nanodiscs embedded in nanowires as optochemical transducers. In: Nanotechnology 22 (27), S. 275505.
