Maoqing Yao

University of Southern California

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Co-Authors

P.D. Dapkus

University of Southern California

Chongwu Zhou

University of Southern California

Michelle L. Povinelli

University of Southern California

Chun-Yung Chi

University of Southern California

Ningfeng Huang

University of Southern California

Stephen B. Cronin

University of Southern California

Shermin Arab

Shahid Rajaee Teacher Training University

Anuj R. Madaria

University of Southern California

K.G. Stephens

University of Surrey

R. F. Peart

University of Surrey

Cooperative Institutions

University of Southern California

Southern California University for Professional Studies

University of Surrey

California Institute of Technology

University of California, Los Angeles

Stanford University

Joint Center for Artificial Photosynthesis

University of Oxford

Chinese Academy of Sciences

Imperial College London

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Scalable synthesis of vertically aligned, catalyst-free gallium arsenide nanowire arrays: towards optimized optical absorption

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE (2012)

Maoqing Yao Anuj R. Madaria Chun-Yung Chi Ningfeng Huang Chenxi Lin

Recently nanostructure materials have emerged as a building block for constructing next generation of photovoltaic devices. Nanowire based semiconductor solar cells, among other candidates, have shown potential to produce high efficiency. In a radial pn junction light absorption and carrier collection can be decoupled. Also nanowires can increase choice of materials one can use to fabricate high efficiency tandem solar cells by relaxing the lattice-match constraint. Here we report synthesis of vertical III-V semiconducting nanowire arrays using Selective-Area Metal Organic Chemical Vapor Deposition (SA-MOCVD) technique, which can find application in various optoelectronic devices. We also demonstrate nanosphere lithography (NSL) patterning techniques to obtain ordered pattern for SAMOCVD. Reflection spectrum of nanowires array made by this technique shows excellent light absorption performance without additional anti-reflection coating layer. Thus, we show that highly ordered nanowire structure is 'not needed' to maximize the absorption in vertical nanowire array. Our scalable approach for synthesis of vertical semiconducting nanowire can have application in high throughput and low cost optoelectronic devices including photovoltaic devices.

10.1117/12.919283

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Nanowires in energy devices

P.D. Dapkus Chun Yung Maoqing Yao Anuj R. Madaria Ting‐Wei Yeh

Semiconductor nanostructures have the potential to make a positive impact on the efficiency and cost of solid state energy devices such as solar cells and light emitting diodes. In this talk I will explore the use of semiconductor nanorods for solar cells and high efficiency LEDs.

Nanorod

Solid-State Lighting

Semiconductor nanostructures

Energy transformation

Solar energy conversion

10.1109/isdrs.2011.6135166

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Optical, electrical, and solar energy-conversion properties of gallium arsenide nanowire-array photoanodes

Energy & Environmental Science (2013)

Shu Hu Chun-Yung Chi Katherine T. Fountaine Maoqing Yao Harry A. Atwater

Periodic arrays of n-GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced near-unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with non-aqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of ∼8.1% under 100 mW cm−2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 ± 15 mV and short-circuit current densities of 24.6 ± 2.0 mA cm−2. The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes.

10.1039/c3ee40243f

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Citations (105)

Enhanced Fabry-Perot resonance in GaAs nanowires through local field enhancement and surface passivation

Nano Research (2014)

Shermin Arab P. Duke Anderson Maoqing Yao Chongwu Zhou P.D. Dapkus

Passivation

10.1007/s12274-014-0477-0

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Large area, low capacitance, GaAs nanowire photodetector with a transparent Schottky collecting junction

Applied Physics Letters (2013)

M. Ashkan Seyedi Maoqing Yao J. O'Brien S. Y. Wang P.D. Dapkus

We present experimental results on a GaAs/Indium-Tin-Oxide Schottky-like heterojunction photodetector based on a nanowire device geometry. By distributing the active detecting area over an array of nanowires, it is possible to achieve large area detection with low capacitance. Devices with bare GaAs and passivated AlGaAs/GaAs nanowires are fabricated to compare the responsivity with and without surface passivation. We are able to achieve responsivity of >0.5A/W and Signal-Noise-Ratio in excess of 7 dB for 2 V applied reverse bias with passivated nanowire devices. Capacitance-voltage measurement yields <5 nF/cm2, which shows a strong possibility for high-speed applications with a broad area device.

Passivation

Indium tin oxide

10.1063/1.4852136

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Doping concentration dependence of the photoluminescence spectra of n-type GaAs nanowires

Applied Physics Letters (2016)

Shermin Arab Maoqing Yao Chongwu Zhou P.D. Dapkus Stephen B. Cronin

In this letter, the photoluminescence spectra of n-type doped GaAs nanowires, grown by the metal organic chemical vapor deposition method, are measured at 4 K and 77 K. Our measurements indicate that an increase in carrier concentration leads to an increase in the complexity of the doping mechanism, which we attribute to the formation of different recombination centers. At high carrier concentrations, we observe a blueshift of the effective band gap energies by up to 25 meV due to the Burstein-Moss shift. Based on the full width at half maximum (FWHM) of the photoluminescence peaks, we estimate the carrier concentrations for these nanowires, which varies from 6 × 1017 cm−3 (lightly doped), to 1.5 × 1018 cm−3 (moderately doped), to 3.5 × 1018 cm−3 (heavily doped) as the partial pressure of the disilane is varied from 0.01 sccm to 1 sccm during the growth process. We find that the growth temperature variation does not affect the radiative recombination mechanism; however, it does lead to a slight enhancement in the optical emission intensities. For GaAs nanowire arrays measured at room temperature, we observe the same general dependence of band gap, FWHM, and carrier concentration on doping.

Blueshift

Disilane

Carrier lifetime

10.1063/1.4947504

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Citations (50)

Tandem Solar Cells Using GaAs Nanowires on Si: Design, Fabrication, and Observation of Voltage Addition

Nano Letters (2015)

Maoqing Yao Sen Cong Shermin Arab Ningfeng Huang Michelle L. Povinelli

Multijunction solar cells provide us a viable approach to achieve efficiencies higher than the Shockley-Queisser limit. Due to their unique optical, electrical, and crystallographic features, semiconductor nanowires are good candidates to achieve monolithic integration of solar cell materials that are not lattice-matched. Here, we report the first realization of nanowire-on-Si tandem cells with the observation of voltage addition of the GaAs nanowire top cell and the Si bottom cell with an open circuit voltage of 0.956 V and an efficiency of 11.4%. Our simulation showed that the current-matching condition plays an important role in the overall efficiency. Furthermore, we characterized GaAs nanowire arrays grown on lattice-mismatched Si substrates and estimated the carrier density using photoluminescence. A low-resistance connecting junction was obtained using n(+)-GaAs/p(+)-Si heterojunction. Finally, we demonstrated tandem solar cells based on top GaAs nanowire array solar cells grown on bottom planar Si solar cells. The reported nanowire-on-Si tandem cell opens up great opportunities for high-efficiency, low-cost multijunction solar cells.

Tandem

Open-circuit voltage

10.1021/acs.nanolett.5b03890

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Citations (122)

Optical and electrical characterization of surface passivated GaAs nanostructures

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE (2014)

Shermin Arab Chun Yung Maoqing Yao Chia‐Chi Chang P.D. Dapkus

GaAs nanostructures are used in different optoelectronic applications including solar cells, LEDs and fast electronics. Although GaAs shows outstanding optical properties, it suffers from surface states and consequently high surface recombination velocity. The surface depletion effects lead to semi-insulating behaviors in GaAs devices. Passivation of GaAs nanostructures (AlGaAs or ionic liquid) lead to surface stability and improvement in optoelectronic properties. We provide a systematic study to compare the optical and electrical improvement after passivation (AlGaAs or ionic liquid) of GaAs nanostructure including nanowires and nanosheets. Both room temperature and low temperature photoluminescent (PL) spectra indicate increase in optical activity of GaAs nanostructures after passivation. Electron beam induced current (EBIC) measurements reveal the diffusion length of carries in different GaAs nanostructures.

Passivation

10.1117/12.2040999

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Scalable Synthesis of Vertically Aligned, Catalyst-Free Gallium Arsenide Nanowire Arrays -- Towards Optimized Optical Absorption and Reflection.

Bulletin of the American Physical Society (2012)

Maoqing Yao Anuj R. Madaria Chun-Yung Chi Chenxi Lin Ningfeng Huang

Reflection

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Control of Modes Coupling in Photonic Crystal Waveguides by Introducing Asymmetry and Long Periodicity

Journal of Nanoscience and Nanotechnology (2010)

Xiaoyu Mao Maoqing Yao Dengke Zhang Yidong Huang Wei Zhang

The control of modes coupling in photonic crystal waveguides (PCWGs) is quite important because it's the basic working mechanism of many devices in optical integrative circuits, such as filters, switches, optical add drop multiplexers (OADMs), etc. Up to now, the researches of this area mostly focus on the modes coupling between two parallel PCWGs or between PCWGs and resonance cavities. In this paper, we proposed a new way of controlling modes coupling in PCWGs by introducing asymmetry and long periodicity. Because of the presence of asymmetry and long periodicity in PCWGs, some interesting modes coupling phenomena, which used to be forbidden in normal PCWGs, happen. Then a filter with a 1.42 nm full-width at the half value (FWHM) and an OADM with a 1.31 nm FWHM and a 0.34 dB insertion loss have been designed by utilizing the new modes coupling phenomena. Our researches not only provide a new way of controlling modes coupling in PCWGs but also benefit the design of many devices in optical integrative circuits greatly.

Mode coupling

10.1166/jnn.2010.2923

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