W. Lu

Deep-level emissions influenced by O and Zn implantations in ZnO

A set of bulk ZnO samples implanted with O and Zn at various densities were investigated by photoluminescence. The implantation concentration of O and Zn is varied between 1x10(17)/cm(3) and 5x10(1 ...

Wide-band “black silicon” based on porous silicon

Solar cells and optical detection devices often incorporate antireflective surfaces to reduce undesired reflection and enhance optical absorption. This letter reports a “black silicon” structure of antireflective porous silicon fabricated by using electrochemical etching. The sample has a gradient-index multilayer structure, i.e., the refraction indices of the structure increase from the top (near the air) to the bottom (near the Si substrate). Reflectance below 5% is obtained over a broad wave number range (3000–28000cm−1) and the depression mechanism of the optical reflectance is analyzed by simulating the structure with the transfer matrix method. The simulated result fits the measured spectra well.

Analysis of temperature dependence of dark current mechanisms for long-wavelength HgCdTe photovoltaic infrared detectors

Resistance-voltage curves of n-on-p Hg1−хCdxTe long-wavelength infrared photodiodes forming 128-element array are measured in the temperature range of 40–150 K. Experimentally obtained characteristics are fitted by the simultaneous-mode nonlinear fitting program. The dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, band-to-band tunneling, and series resistance effect are included in the physical model for R-V curve fitting. Six characteristic parameters as function of temperature are extracted from measured R-V curves. The characteristics of extracted current components at low temperatures indicate significant contributions from tunneling effects, which is the dominant leakage current mechanism for reverse bias greater than approximately 50 mV. The Hg-vacancy-induced acceptor trap tends to invert to donor type at higher temperature, typically larger than 120 K, while it can maintain stable at the temperature of 60–40 K. The stable temperature of ion-implanta...

Intergrain current flow in a randomly oriented polycrystalline SmFeAsO0.85 oxypnictide

We report a direct current transport study of the local intergrain connections in a polycrystalline SmFeAsO0.85 (Sm1111) bulk, for which we earlier estimated significant intergranular critical current density Jc. Our combined low temperature laser scanning microscopy and scanning electron microscopy observations revealed only few grain-to-grain transport current paths, most of which switched off when a magnetic field was applied. These regions typically occur where current crosses Fe–As, which is a normal-metal wetting-phase that surrounds Sm1111 grains, producing a dense array of superconducting-normal-superconducting contacts. Our study points out the need to reduce the amount of grain boundary-wetting Fe–As phase, as well as the crack density within pnictide grains, as these defects produce a multiply connected current-blocking network.

Self-heating simulation of GaN-based metal-oxide-semiconductor high-electron-mobility transistors including hot electron and quantum effects

Undoped GaN-based metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) with atomic-layer-deposited Al2O3 gate dielectrics are fabricated with gate lengths from 1 up to 40μm. Using a two-dimensional numerical simulator, we report the results of self-heating simulations of the GaN-based MOS-HEMTs, including hot electron and quantum effects. The simulated electrical characteristics are in good agreement with reported experimental data. The effect of the gate and source/drain extension lengths on both the output performance and self-heating is discussed in detail, allowing for device optimization. The dissipated Joule electric power causes the self-heating effects, which lead to negative differential output conductance. Our results demonstrate that the hot electrons make a negligible contribution to the negative differential output conductance in our long channel MOS-HEMTs. In order to investigate their joint interactions to the MOS-HEMT’s operation, the different static interface trap and...

A hybrid surface passivation on HgCdTe long wave infrared detector with in-situ CdTe deposition and high-density hydrogen plasma modification

A hybrid surface passivation, in-situ CdTe passivation and high-density hydrogen plasma modification, is used to improve the surface quality of typical n+-on-p HgCdTe long wave infrared photodiode detectors. Three types of surface-passivated pixels, conventional, in-situ CdTe, and hybrid surface passivation, are fabricated in one chip for better comparison. The maximum dynamic resistances of the hybrid-surface-passivation device are increased to 1∼2 times greater than that in the conventional surface passivation technique. Theoretical modeling shows that the hybrid passivation can significantly suppress the trap-assisted tunneling current. Shallow traps close to the Fermi level under reverse voltage, which are the main source of the trap-assisted tunneling current for conventional surface passivation processing, are reduced by the hybrid passivation treatment.

Low-temperature interface engineering for high-quality ZnO epitaxy on Si(111) substrate

ZnO(0001)∕Si(111) interface is engineered by using a three-step technique, involving low-temperature Mg deposition, oxidation, and MgO homoepitaxy. The double heterostructure of MgO(111)∕Mg(0001)∕Si(111) formed at −10°C prevents the Si surface from oxidation and serves as an excellent template for single-domain ZnO epitaxy, which is confirmed with in situ reflection high-energy electron diffraction observation and ex situ characterization by transmission electron microscopy, x-ray diffraction, and photoluminescence. The low-temperature interface engineering method can also be applied to control other reactive metal/Si interfaces and obtain high-quality oxide templates accordingly.

Pressure effect on superconducting properties of LaO1-xFxFeAs (x=0.11) superconductor

Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03u2009GPa) were carried out on a newly discovered Fe-based superconductor LaO1-xFxFeAsxa0(x=0.11). The transition temperature TC, defined as a point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dTC/dP of about 1.2u2009Ku2009GPa−1. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing the low TC superconducting phase toward the main (optimal) superconducting phase.

Voltage tunable two-color InAs∕GaAs quantum dot infrared photodetector

We report a bias voltage tunable two-color InAs/GaAs quantum dot infrared photodetector working under the normal incidence infared irradiation. The two-color detection of our device is realized by combining a photovoltaic and a photoconductive response by bias voltage tuning. The photovoltaic response is attributed to the transition of electron from the ground state to a high continuum state. The photoconductive response arises from the transition of electron from the ground state to the wetting layer state through the barrier via Fowler-Nordheim tunneling evidenced by a broad feature of the photocurrent peak on the high energy side

Carrier concentration and mobility in GaN epilayers on sapphire substrate studied by infrared reflection spectroscopy

We report the measurement of carrier concentration and mobility of metalorganic chemical vapor deposited GaN thin films on the sapphire substrate by an infrared reflection technique. By fitting with the experimental data we obtain all the parameters of the lattice vibration oscillators and of the plasmon. From the plasmon frequency and the damping constant we have derived the carrier concentration and the electron mobility. The concentration agrees with the Hall data very well while the mobility values are smaller than that of the Hall measurement by a factor of about 0.5. We attribute such mobility lowering to the increase of scattering for the electrons coupling with the incident photons.