Ch. Y. Wang

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

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Publications (23)31.94 Total impact

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    ABSTRACT: The influence of metalorganic chemical vapor deposition growth conditions on the indium oxide surface properties is investigated using photoelectron spectroscopy. Particular attention is paid to nanocrystalline samples grown at fairly low temperatures which are known for their high sensitivity to ozone. The results are compared to measurements on In2O3 films in cubic and rhombohedral crystal structure. It is shown that the growth conditions have a strong impact on the physical properties and that samples grown at 200 °C or below are highly oxygen-deficient and rich in defects, influencing the surface chemical and electronic properties and resulting in high ozone sensitivity.
    Journal of Applied Physics 05/2012; 111(9). · 2.21 Impact Factor
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    ABSTRACT: In our previous work (part I), a compact energy-saving photostimulated ozone sensor has been demonstrated, which can monitor a wide concentration range of ozone in synthesized air. However, for practical purposes such as environmental ozone detection in air or the monitoring of ozone concentration for water treatment applications, the interference of water vapor/humidity must be considered, because it has a large influence on the sensor performance. In the present work, the photostimulated ozone sensor has been used to detect ozone in a high-humidity gas environment. Sensor signals were found to be reproducible. By sealing the sensor chamber with a porous hydrophobic membrane, which prevented the passage of liquid water, submerged measurements were carried out. A mechanism of the influence of humidity on the photostimulated sensor has been proposed based on electrical measurements.
    Sensors and Actuators B: Chemical. 03/2012; 164(1):37–42.
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    ABSTRACT: In this work, we report on the epitaxial growth of high-quality cubic indium oxide thick films on c-plane sapphire substrates using a two-step growth process. The epitaxial relationship of In2O3 on (0001) Al2O3 has been investigated. The (222) plane spacing and lattice parameter of a most strain-relaxed high-quality In2O3 film have been determined to be 292.58 pm and 1013.53 pm, respectively. The electronic properties in dependence of the film thickness are interpreted using a three-region model. The density at the surface and interface totals (3.3±1.5)×1013cm-2, while the background electron density in the bulk was determined to be (2.4±0.5)×1018cm-3. Furthermore, post treatments such as irradiation via ultraviolet light and ozone oxidation have been found to influence only the surface layer, while the bulk electronic properties remain unchanged.
    Journal of Applied Physics 11/2011; 110(9). · 2.21 Impact Factor
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    ABSTRACT: Normally carbon monoxide is considered to be a reducing agent, which can be used for CO detection. However, oxidizing behaviour of CO, to the best of our knowledge, has not been reported. In this work, we find that CO can also act as oxidizing gas at room temperature on photostimulated ZnO nanowires. For photostimulation, the low intensity of light emitting diodes is used to induce a very low photocurrent, which, however, leads to a large resistance change due to the ultraviolet light interaction with the nanowire surface. During CO detection, the oxygen vacancies enhance the CO adsorption on the nanowire surface even at room temperature. CO molecules adsorbing on the surface are presumably bound to a zinc–oxygen vacancy complex, causing a reduction in free electron concentration, which leads to an oxidizing effect of CO gas. An adsorption model for CO is proposed to explain the experimental observations. These results shed light on developing room-temperature CO sensor using oxide nanostructures with the help of high energetic photon replacing the high-temperature process normally used in traditional gas sensors.
    Journal of Physics D Applied Physics 07/2011; 44(30):305302. · 2.53 Impact Factor
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    ABSTRACT: Ozone sensors are of great demand for monitoring high-concentration ozone for industrial applications and low-concentration ozone for protecting people's health, although commercial ozone sensors are limited in their detection range. In this work, it is demonstrated that compact energy-saving photostimulated ozone sensors based on indium oxide nanoparticles can detect ozone with a dynamical range over four orders of magnitude at room temperature. The photostimulated ozone sensor shows a very low cross response to NO2, CO, and CO2. Furthermore, the sensing signal is very reproducible, and no hysteresis effects were found in repeated measurements.
    Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2011; 152(2):235-240.
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    ABSTRACT: The bulk and surface electronic structure of In 2 O 3 has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline In 2 O 3 is determined as 2.93 0.15 and 3.02 0.15 eV for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is investigated from x-ray photoemission spectroscopy measurements and density-functional theory calculations. These show excellent agreement, supporting the absence of any signifi-cant indirect nature of the In 2 O 3 band gap. Clear experimental evidence for an s-d coupling between In 4d and O 2s derived states is also observed. Electron accumulation, recently reported at the 001 surface of bixbyite material, is also shown to be present at the bixbyite 111 surface and the 0001 surface of rhombohedral In 2 O 3 .
    01/2009; 7960.
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    ABSTRACT: The bulk and surface electronic structure of In2O3 has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline In2O3 is determined as 2.93±0.15 and 3.02±0.15eV for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is investigated from x-ray photoemission spectroscopy measurements and density-functional theory calculations. These show excellent agreement, supporting the absence of any significant indirect nature of the In2O3 band gap. Clear experimental evidence for an s-d coupling between In4d and O2s derived states is also observed. Electron accumulation, recently reported at the (001) surface of bixbyite material, is also shown to be present at the bixbyite (111) surface and the (0001) surface of rhombohedral In2O3 .
    Physical review. B, Condensed matter 01/2009; 79(20).
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    ABSTRACT: We report on the phase stabilization of rhombohedral (rh-) In2O3 films on sapphire substrate deposited by metal organic chemical vapor deposition. With the help of a high-temperature nucleation layer and evolutionary structural selection of rhombohedral phase during the growth process, stable epitaxial growth of single crystalline rh-In2O3 is achieved. The mechanism of phase selective epitaxial growth is studied by means of high-resolution X-ray diffraction and transmission electron microscopy measurements. Furthermore, Raman spectroscopy measurements are carried out to investigate the phonon properties of rh-In2O3. Raman-active phonon modes of rh-In2O3 are first identified.
    Crystal Growth & Design - CRYST GROWTH DES. 02/2008; 8(4).
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    ABSTRACT: There is a high demand for compact low-cost ozone (O3) sensors. It has been shown that indium oxide (In2O3) thin films grown by metal-organic vapor-phase epitaxy (MOVPE) act as an O3 sensitive material, which can be activated at roomtemperature by ultraviolet (UV) light. In the present work we integrated the In2O3 sensing thin film and an (AlGaIn)N based near-UV LED back-to-back on a single sensor chip. The integrated In2O3 film-LED sensor was exposed to varying ozone concentrations ranging from 38 ppb to 726 ppb and found to be sensitive to even the lowest O3 concentration.
    Proc SPIE 02/2008;
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    ABSTRACT: The occurrence of cubic indium nitride thin layers grown by molecular beam epitaxy on top of c-plane sapphire substrates modified by an intermediate layer of cubic indium oxide is reported. An orientation relationship between the (0001) plane of Al2O3 and both (001) surfaces of body-centered cubic In2O3 and zinc-blende InN is demonstrated by means of electron and X-ray diffraction and by transmission electron microscopy. We propose that the demonstrated approach is able to stabilize the non equilibrium phase of InN (i. e., the cubic polytype) due to a low lattice mismatch together with a four fold surface atomic arrangement of the indium oxide-indium nitride interface. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2008; 5(2):514 - 517.
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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: In2O3 nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. The response of 10-nm thick In2O3 particle containing layers to NOx and O2 gases is investigated. The lowest detectable NOx concentration is ∼200 ppb and the sensor performance is strongly dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NOx is found to be above 104. Furthermore, the cross-sensitivity against O2 is very low, demonstrating that the In2O3 nanoparticles are very suitable for the selective NOx detection.
    Sensors and Actuators B: Chemical. 01/2008;
  • G. Ecke, Ch. Y. Wang, V. Cimalla, O. Ambacher
    Physica Status Solidi (c). 01/2008; 5(6):1603-1605.
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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. · 2.21 Impact Factor
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    ABSTRACT: There is a high demand for compact low-cost ozone sensors. It has been shown recently that In2O3 nanolayers can act as ozone sensitive films activated at room temperature by ultraviolet light. In the present work, the authors integrate ultrathin layers of In2O3 nanoparticles and a GaInN/GaN based blue light emitting diode (LED) on a single sensor chip. The integrated sensor was found to be sensitive to O3 concentrations as low as ∼ 40 ppb. These results demonstrate that by integrating GaInN/GaN based blue LEDs and metal oxide sensing layers back to back on a single chip, compact and robust gas sensors can be realized.
    Applied Physics Letters 09/2007; 91(10):103509-103509-3. · 3.79 Impact Factor
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    ABSTRACT: In <sub>2</sub>O<sub>3</sub> nanoparticles were synthesized at low substrate temperatures by the metal organic chemical vapor deposition technique. Nanoparticles with a mean diameter from 3 to 33 nm can be obtained by varying the growth temperature. Photoreduction and oxidation studies were carried out for particle-containing layers exhibiting a resistance change of more than five orders of magnitude after ultraviolet irradiation and oxidation by ozone. A grain boundary model was proposed to understand the photoreduction and oxidation mechanism for the nanoparticle layers. It was suggested that by photoreduction the nanoparticles are reactivated throughout the layer. The Schottky barrier between the nanoparticles decreases inducing a reduction of the space-charge-limited region. After oxidation, a completely depleted space-charge region covering the whole volume of In <sub>2</sub>O<sub>3</sub> nanoparticles is formed. Furthermore, the bulk diffusion process dominates the response of thick layers during the oxidation process. By decreasing the layer thickness down to 10 nm, surface effects dominate, resulting in an ultrafast response to changes in ozone concentration. The typical response time of very thin In <sub>2</sub>O<sub>3</sub> nanoparticle layers was determined to be less than 1 s.
    Journal of Applied Physics 09/2007; · 2.21 Impact Factor
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    ABSTRACT: Single crystalline In2O3 is a prospective material to be used as a gate dielectric in InN based field effect transistors (FETs). This work addresses structural investigations of In2O3/InN heterostuctures for metal-oxide-semiconductor FET devices. Single crystalline cubic In2O3 (111) films were epitaxially grown on hexagonal InN (0001) epilayers. The epitaxial relationship between the film and the template was determined as In2O3[111]‖InN[0001] and In2O3[10]‖InN〈110〉 with an effective lattice mismatch of 2.14%. On the basis of the structural investigations, a phenomenological model for the growth of In2O3 on InN (0001) is proposed.
    Applied Physics Letters 05/2007; 90(22):221902-221902-3. · 3.79 Impact Factor
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    ABSTRACT: A room temperature ozone induced oxidation of thin InN films is proposed to improve the electric transport properties. The sheet carrier density is reduced upon oxidation by a value which is in the order of the electron concentration of an untreated InN surface. Thus, ozone effectively passivates the surface defect states on InN and might be an effective method to prepare InN films for electronic applications. A model for the improved electron transport properties is proposed taking into account the decreased surface band bending and the decreased influence of surface electrons on the net mobility of InN layers.
    Applied Physics Letters 05/2007; · 3.79 Impact Factor
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    ABSTRACT: Cubic InN layers were grown by molecular beam epitaxy on buffer layers of indium oxide prepared onto sapphire (0001) substrates. The structure was analyzed by means of electron diffraction and transmission electron microscopy. The intermediate indium oxide layer presents a body centered cubic (bcc) structure, with bcc-In2O3(001)‖Al2O3(0001) plane relationship. Thereupon, a zinc-blende phase of InN (001) was grown with a reticular misfit of 1.6% and a significant reduction of mismatch-related defects. This good coherence offers a promising expectation to obtain high quality cubic InN layers superior to other highly mismatched cubic substrates used previously.
    Applied Physics Letters 02/2007; 90(9):091901-091901-3. · 3.79 Impact Factor
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    ABSTRACT: Tuning of structural and electrical properties of indium oxide (In2O3) films by means of metal organic chemical vapor deposition is demonstrated. Phase selective growth of rhombohedral In2O3(0001) and body-centered cubic In2O3(001) polytypes on (0001) sapphire substrates was obtained by adjusting the substrate temperature and trimethylindium flow rate. The specific resistance of the as-grown films can be tuned by about two orders of magnitude by varying the growth conditions.
    Thin Solid Films 01/2007; 515(16):6611-6614. · 1.60 Impact Factor

Publication Stats

38 Citations
40 Downloads
754 Views
31.94 Total Impact Points

Institutions

  • 2009–2012
    • Fraunhofer Institute for Applied Solid State Physics IAF
      Freiburg, Baden-Württemberg, Germany
  • 2006–2008
    • Technische Universität Ilmenau
      • Institut für Mikro- und Nanotechnologien
      Ilmenau, Thuringia, Germany