Wendao Xu

ZnO light-emitting diode grown by plasma-assisted metal organic chemical vapor deposition

We report a breakthrough in fabricating ZnO homojunction light-emitting diode by metal organic chemical vapor deposition. Using NO plasma, we are able to grow p-type ZnO thin films on n-type bulk ZnO substrates. The as-grown films on glass substrates show hole concentration of 1016–1017cm−3 and mobility of 1–10cm2V−1s−1. Room-temperature photoluminescence spectra reveal nitrogen-related emissions. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3V. Electroluminescence at room temperature has been demonstrated with band-to-band emission at I=40mA and defect-related emissions in the blue-yellow spectrum range.

Dopant source choice for formation of p-type ZnO: Li acceptor

Li-doped, p-type ZnO thin films have been realized via dc reactive magnetron sputtering. An optimized result with a resistivity of 16.4Ωcm, Hall mobility of 2.65cm2∕Vs, and hole concentration of 1.44×1017cm−3 was achieved, and electrically stable over a month. Hall-effect measurements supported by secondary ion mass spectroscopy indicated that the substrate temperature played a key role in optimizing the p-type conduction of Li-doped ZnO thin films. Furthermore, ZnO-based p-n homojunction was fabricated by deposition of a Li-doped p-type ZnO layer on an Al-doped n-type ZnO layer.

Identification of acceptor states in Li-doped p-type ZnO thin films

We investigate photoluminescence from reproducible Li-doped p-type ZnO thin films prepared by dc reactive magnetron sputtering. The LiZn acceptor state, with an energy level located at 150meV above the valence band maximum, is identified from free-to-neutral-acceptor transitions. Another deeper acceptor state located at 250meV emerges with the increased Li concentration. A broad emission centered at 2.96eV is attributed to a donor-acceptor pair recombination involving zinc vacancy. In addition, two chemical bonding states of Li, evident in x-ray photoelectron spectroscopy, are probably associated with the two acceptor states observed.

p-type behavior in nominally undoped ZnO thin films by oxygen plasma growth

We report on intrinsic p-type ZnO thin films by plasma-assisted metal-organic chemical vapor deposition. The optimal results give a resistivity of 12.7Ωcm, a Hall mobility of 2.6cm2∕Vs, and a hole concentration of 1.88×1017cm−3. The oxygen concentration is increased in the intrinsic p-type ZnO, compared with the n-type layer. Two acceptor states, with the energy levels located at 160 and 270meV above the valence band maximum, are identified by temperature-dependent photoluminescence. The origin of intrinsic p-type behavior has been ascribed to the formation of zinc vacancy and some complex acceptor center.

High-performance terahertz wave absorbers made of silicon-based metamaterials

Electromagnetic (EM) wave absorbers with high efficiency in different frequency bands have been extensively investigated for various applications. In this paper, we propose an ultra-broadband and polarization-insensitive terahertz metamaterial absorber based on a patterned lossy silicon substrate. Experimentally, a large absorption efficiency more than 95% in a frequency range of 0.9–2.5 THz was obtained up to a wave incident angle as large as 70°. Much broader absorption bandwidth and excellent oblique incidence absorption performance are numerically demonstrated. The underlying mechanisms due to the combination of a waveguide cavity mode and impedance-matched diffraction are analyzed in terms of the field patterns and the scattering features. The monolithic THz absorber proposed here may find important applications in EM energy harvesting systems such as THz barometer or biosensor.

Discrimination of Transgenic Rice containing the Cry1Ab Protein using Terahertz Spectroscopy and Chemometrics.

Spectroscopic techniques combined with chemometrics methods have proven to be effective tools for the discrimination of objects with similar properties. In this work, terahertz time-domain spectroscopy (THz-TDS) combined with discriminate analysis (DA) and principal component analysis (PCA) with derivative pretreatments was performed to differentiate transgenic rice (Hua Hui 1, containing the Cry1Ab protein) from its parent (Ming Hui 63). Both rice samples and the Cry1Ab protein were ground and pressed into pellets for terahertz (THz) measurements. The resulting time-domain spectra were transformed into frequency-domain spectra, and then, the transmittances of the rice and Cry1Ab protein were calculated. By applying the first derivative of the THz spectra in conjunction with the DA model, the discrimination of transgenic from non-transgenic rice was possible with accuracies up to 89.4% and 85.0% for the calibration set and validation set, respectively. The results indicated that THz spectroscopic techniques and chemometrics methods could be new feasible ways to differentiate transgenic rice.

Dual-acceptor p-type behaviour in ZnO films grown by plasma-assisted metalorganic chemical vapour deposition

Nominally undoped and N-doped ZnO thin films were grown by plasma-assisted metalorganic chemical vapour deposition. P-type conductivity was confirmed by Hall-effect measurements, not only in the N-doped but also in the nominally undoped ZnO. The zinc vacancy and extrinsic nitrogen acceptor states were identified by low-temperature photoluminescence, with the energy level located at 270 meV and 180 meV above the valence-band maximum, respectively. An evident increment in the oxygen as well as nitrogen concentration in the p-type ZnO : N layer was well confirmed by secondary ion mass spectroscopy.

Terahertz sensing of chlorpyrifos-methyl using metamaterials.

By squeezing electromagnetic energy into small volumes near a metal-dielectric interface, plasmonics provide many routes to enhance and manipulate light-matter interactions, which presents new strategies for signal enhancing technologies. As an extension of the ideas of plasmonics to the terahertz (THz) range, metamaterials have shown great potential in sensing applications. In this study, terahertz time-domain spectroscopy (THz-TDS) combined with metamaterials was used to detect chlorpyrifos-methyl (CM), which is one type of the broad-spectrum organophosphorus pesticides. The results demonstrate that sensitivity is greatly improved using THz metamaterials, with the limit of detection (LOD) of CM reaching 0.204mgL-1, which is lower than the World Health Organizations provisional guideline limit for CM in vegetables (1mgL-1). The results indicated that THz spectroscopy combined with metamaterials could be a valuable method for highly sensitive THz applications, presenting a new strategy for food quality and safety control in the future.

Modulation of far-infrared light transmission by graphene-silicon Schottky junction

Tunable conductive properties of graphene in terahertz and far-infrared regimes provide a prominent way to control electromagnetic waves. In this paper, we explore the photon-electric properties of the graphene-silicon heterostructure and its application in modulating the transmission of far-infrared light. Experimentally, we show this structure will give rise to different transmission modulation ratios with amplitudes varying from tens to few percentages, dependent on the operation wavelength. The modulation effect gradually decreases and saturates within wavenumbers 1000-2000 cm−1 influenced by the pump light power. The diode transmission behavior is explicitly interpreted in terms of the Schottky junction formed between the graphene-silicon interface. The results give a further deep understanding of the electromagnetic behavior of graphene in the far-infrared region that may be integrated for potential applications.