Xingyou Chen

Preparation and photoabsorption characterization of BiFeO3 nanowires

Perovskite-type polycrystalline BiFeO3 (BFO) nanowires (∼50nm in diameter and ∼5μm in length) were synthesized using the anodized alumina template technique. An energy band gap of ∼2.5eV was determined from the UV-visible diffuse reflectance spectrum, and its photocatalytic ability to produce O2 was revealed under UV irradiation. Weak ferromagnetism at room temperature and superparamagnetism at low temperature were observed for the BFO nanowires, different from the antiferromagnetic order in bulk BFO, reflecting the significant size effects on the magnetic ordering of BFO.

Effect of compressive stress on stability of N-doped p-type ZnO

Nitrogen-doped p-type zinc oxide (p-ZnO:N) thin films were fabricated on a-/c-plane sapphire (a-/c-Al2O3) by plasma-assisted molecular beam epitaxy. Hall-effect measurements show that the p-type ZnO:N on c-Al2O3 degenerated into n-type after a preservation time; however, the one grown on a-Al2O3 showed good stability. The conversion of conductivity in the one grown on c-Al2O3 ascribed to the faster disappearance of NO and the growing N2(O), which is demonstrated by x-ray photoelectron spectroscopy (XPS). Compressive stress, caused by lattice misfit, was revealed by Raman spectra and optical absorption spectra, and it was regarded as the root of the instability in ZnO:N.

2.4 µm InP-based antimony-free triangular quantum well lasers in continuous-wave operation above room temperature

In this work, we report on the above-room-temperature continuous-wave operation of InP-based antimony-free triangular quantum well (QW) lasers emitting up to approximately 2.4 µm. X-ray diffraction measurement confirms the favorable structural quality of the highly strained triangular QWs composed of InAs/In0.53Ga0.47As digital alloy. The maximum continuous-wave working temperature reaches 340 K, and the output power at 300 K is about 11 mW/facet at an injection current of 350 mA. The internal quantum efficiency of 58% is deduced at 300 K, and the extrapolated threshold current density for infinite cavity length is 210 A/cm2 per quantum well.

Dark current suppression in metamorphic In0.83Ga0.17As photodetectors with In0.66Ga0.34As/InAs superlattice electron barrier

We report on InP-based metamorphic In0.83Ga0.17As photodetectors with dramatically suppressed dark currents by inserting a strain-compensated In0.66Ga0.34As/InAs superlattice electron barrier in the In0.83Ga0.17As absorption layer. Compared with a reference detector without the barrier, the device showed that the dark current is reduced by about half at room temperature and is more than two orders of magnitude at 77 K at low bias, while the responsivity remained unchanged. The generation recombination and tunneling currents are significantly suppressed, and the dark current remains diffusion-current-limited above 175 K. The dark current density at −10 mV is reduced to 3.95 nA/cm2 at 175 K.

Tailoring the performances of low operating voltage InAlAs/InGaAs avalanche photodetectors

We present tailoring of the performances of thin multiplication layer InAlAs/InGaAs avalanche photodetectors (APDs) with operating voltages lower than 20 V. Their operating voltages, gain-voltage slopes and dark currents were successfully tailored by changing the electric field distributions in avalanche region. The thin multiplication layer APDs show small activation energies of the dark current ranging from 0.12 to 0.19 eV at temperatures above 220 K, suggesting a band-trap-band tunneling dominant dark current mechanism over this temperature range. The dark currents show very weak temperature dependences at temperatures lower than 175 K, which mainly originate from the band-to-band tunneling and the surface leakage currents. The spectral responsivity of those APDs show anomalous negative temperature coefficients at gain factors larger than 1, which is attributed to the enhanced phonon scattering effect of carriers in the avalanche region at higher temperatures. Good gain factor uniformity at a given bias is observed for those APDs, and the charge layer is found to help improve the gain uniformity.

Low Operating Voltage and Small Gain Slope of InGaAs APDs With p-Type Multiplication Layer

We reported separate absorption and multiplication InAlAs/InGaAs avalanche photodiodes with a p-type multiplication layer. Wedge-shaped electric field profiles with different gradients and peak intensities confined in a thin InAlAs avalanche layer were realized. These devices showed optimum operating gains up to 40 in linear mode with low operating voltages <;20 V, small gain slopes, and high-gain uniformity. Moreover, a reduced breakdown voltage temperature coefficient <;6 mV/K in the temperature range of 200-350 K was observed, whereas the dark current showed a noticeable increase. Those multiplication performances are attributed to the modified electric field profiles and are ideally suitable for focal plane array imaging applications.

Improved Performance of 2.2-

InP-based InAs/InGaAs quantum well (QW) lasers emitting at wavelength about 2.2 μm have been demonstrated. To study the effects of triangular QWs on laser performance, lasers grown with digital alloy triangular QWs are discussed and rectangular InAs QW lasers are presented for reference. The use of triangular QWs improves laser performance in terms of threshold current density, output power, characteristic temperature, maximum operation temperature, quantum efficiency, and internal optical loss coefficient. By using triangular QWs, the threshold current density decreases from 2.58 to 1.42 kA/cm2 under continuous-wave driving condition at 300 K, and the output power increases from 3.6 to 10.4 mW/facet at an injection current of 400 mA. The maximum operation temperature reaches up to 330 K, about 20 K higher than the value of the laser with rectangular-shaped QWs.

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The carrier scattering and relaxation dynamics in an n-type In0.83Ga0.17As ternary alloy are investigated by measuring the temperature dependent electron Hall mobilities and the hole lifetimes as a function of doping density. The dominant scattering mechanisms in temperature ranges of T < 80 K, 80 < T < 120 K, and 120 < T < 300 K in lightly doped In0.83Ga0.17As are found to be impurity scattering, alloy disorder scattering and phonon scattering, respectively, while in heavily doped In0.83Ga0.17As alloy scattering dominates over the whole measured temperature range. By fitting the measured temperature dependent carrier lifetimes, the dominant carrier relaxation mechanisms in lightly doped In0.83Ga0.17As are identified to be the radiative recombination and the Shockley–Read–Hall effect for samples grown on InP and GaAs substrates, respectively. The lifetime in heavily doped In0.83Ga0.17As is below 10 ns with an Auger dominated recombination. Lastly, photoluminescence as well as the light absorption measurements are performed, showing that the grown lightly doped In0.83Ga0.17As has a high optical quality comparable to the lattice-matched In0.53Ga0.47As.

InAs/InGaAs QW Lasers on InP by Using Triangular Wells

We report the effect of Bi surfactant on the properties of highly strained InAs/InGaAs triangular quantum wells grown on InP substrates. Reduced surface roughness, improved heterostructure interfaces and enhanced photoluminescence intensity at 2.2 μm are observed by moderate Bi-mediated growth. The nonradiative processes are analysed based on temperature-dependent photoluminescence. It is confirmed that Bi incorporation is insignificant in the samples, whereas excessive Bi flux during the growth results in deteriorated performance. The surfactant effect of Bi is promising to improve InP-based highly strained structures while the excess of Bi flux needs to be avoided.

Carrier scattering and relaxation dynamics in n-type In0.83Ga0.17As as a function of temperature and doping density

Nitrogen-doped ZnO thin films were grown on c-plane sapphire (Al2O3) substrates via plasma-assisted molecular beam epitaxy using plasma activated nitric oxide (NO) as the oxygen source and dopant. X-ray diffraction measurements indicate that a small NO flux benefits the crystal quality of the thin films. Hall effect measurements indicate that the electron density of the ZnO films decreases gradually with decreasing NO flux, and the conduction reverses to p-type at a certain flux. Optical emission spectra indicate that the N atom content in the NO plasma increases with decreasing NO flux, and the origin of this is discussed. X-ray photoelectron spectroscopy measurements demonstrate that the number of N atom occupied O sites in the ZnO lattice increases correspondingly.