Wenwei Liu

Investigation on the structural origin of n-type conductivity in InN films

This work presents a study of the correlation between the electrical properties and the structural defects in nominally undoped InN films. It is found that the density of edge-type threading dislocations (TDs) considerably affects the electron concentration and mobility in InN films. The Hall-effect measured electron concentration increases, while the Hall mobility decreases with the increase in the edge-type TD density. With the combination of secondary ion mass spectrometry and positron annihilation analysis, we suggest that donor-type point defects at the edge-type TD lines may serve as dominant donors in InN films and affect the carrier mobility.

Stable multiplication gain in GaN p?i?n avalanche photodiodes with large device area

Visible-blind p-i-n avalanche photodiodes (APDs) were fabricated with high-quality GaN epilayers deposited on c-plane sapphire substrates by metal-organic chemical vapour deposition. Due to low dislocation density and a sophisticated device fabrication process, the dark current was as small as similar to 0.05 nA under reverse bias up to 20V for devices with a large diameter of 200 mu m, which was among the largest device area for GaN-based p-i-n APDs yet reported. When the reverse bias exceeded 38V the dark current increased sharply, exhibiting a bulk avalanche field-dominated stable breakdown without microplasma formation or sidewall breakdown. With ultraviolet illumination (360 nm) an avalanche multiplication gain of 57 was achieved.

Ultrafast photovoltaic effects in miscut Nb-doped SrTiO3 single crystals

Picosecond photovoltaic effect in miscut Nb-doped SrTiO3 single crystal has been observed under ultraviolet pulsed laser irradiation at ambient temperature without an applied bias. The 10%–90% rise time and the full width at half maximum are 828 and 670 ps, respectively, which is faster than that of exact cut Nb-doped SrTiO3 single crystal. A model based on terrace structure is put forward to explain the observation.

Effect of annealing on photoluminescence properties of neon implanted GaN

The effect of thermal annealing on the luminescence properties of neon implanted GaN thin films was studied. Low temperature photoluminescence (PL) measurements were carried out on the samples implanted with different doses ranging from 10(14) to 9 x 10(15) cm(-2) and annealed isochronally at 800 and 900 degrees C. We observed a new peak appearing at 3.44 eV in the low temperative PL spectra of all the implanted samples after annealing at 900 degrees C. This peak has not been observed in the PL spectra of implanted samples annealed at 800 degrees C except for the samples implanted with the highest dose. The intensity of the yellow luminescence (YL) band noticed in the PL spectra measured after annealing was observed to decrease with the increase in dose until it was completely suppressed at a dose of 5 x 10(15) cm(-2). The appearance of a new peak at 3.44 eV and dose dependent suppression of the YL band are attributed to the dissociation of VGaON complexes caused by high energy ion implantation.

Photoelectric characteristics of metal/InGaN/GaN heterojunction structure

A heterojunction structure photodetector was fabricated by evaporating a semitransparent Ni/Au metal film on the InGaN/GaN structure. The photocurrent (PC) spectra show that both the Schottky junction (NiAu/InGaN) and the InGaN/GaN isotype heterojunction contribute to the PC signal which suggests that two junctions are connected in series and result in a broader spectral response of the device. Secondary electron, cathodoluminescence and electron-beam-induced current images measured from the same area of the edge surface clearly reveal the profile of the layer structure and distribution of the built-in electric field around the two junctions. A band diagram of the device is drawn based on the consideration of the polarization effect at the InGaN/GaN interface. The analysis is consistent with the physical mechanism of a tandem structure of two junctions connected in series.

Structural properties of Ne implanted GaN

We report a study on the micro-structural changes in GaN due to neon ion implantation using the x-ray diffraction and Raman scattering techniques. An implantation dose of 10(14) cm(-2) was found unable to produce lattice deformation observable by Raman measurements. For higher doses of implantation several disorder activated Raman scattering centers were observed which corroborate the literature. A new dose dependent feature has been recorded at 1595 cm(-1) for higher implantation doses which is suggested to be the vibrational mode of microcavities produced in the lattice.

Lateral Infrared Photovoltaic Effects in Ag-Doped ZnO Thin Films

A transient lateral photovoltaic effect has been observed in Ag-doped ZnO thin films. Under the nonuniform irradiation of a 1064 nm pulsed laser, the photovoltaic response shows high sensitivity to the spot position on the film surface. The highest photovoltaic responsivity of 27.1 mV mJ(-1) was observed, with a decline time of similar to 1.5 ns and a full width at half-maximum (FWHM) of similar to 4 ns. The photovoltaic position sensitivity can reach about 3.8 mV mJ(-1) mm(-1). This paper demonstrates the potential of Ag-doped ZnO films in the position-sensitive infrared detection

Lateral photovoltage of B-doped ZnO thin films induced by 10.6 mu m CO(2) laser

B-doped ZnO thin films were observed to have a lateral laser-induced photovoltaic effect: the saturation value varied very linearly with the 10.6 mu m constant laser spot position between the electrodes on the ZnO surface. It was found that the temperature gradient in the direction of electron transfer (along the film surface) due to the laser spot causes this photovoltage signal to be linearly dependent on the position of the laser spot in this isotropic system. This linearity is expected to make ZnO a candidate for position-sensitive photodetectors.

Suppression of indium droplet formation by adding CCl4 during metalorganic chemical vapor deposition growth of InN films

In this work, the influences of CCl4 on the metalorganic chemical vapor deposition (MOCVD) growth of InN were studied for the first time. It was found that the addition of CCl4 can effectively suppress the formation of metal indium (In) droplets during InN growth, which was ascribed to the etching effect of Cl to In. However, with increasing of CCl4 flow, the InN growth rate decreased but the lateral growth of InN islands was enhanced. This provides a possibility of promoting islands coalescence toward a smooth surface of the InN film by MOCVD. The influence of addition of CCl4 on the electrical properties was also investigated.

Boron-doped ZnO for infrared detection

Boron-doped ZnO thin film was fabricated on fused quartz substrate using the pulse laser deposition method. The infrared photovoltaic properties were studied using Nd:YAG 1.064μm pulse laser and 10.6μm carbon dioxide continuous laser. When the film was irradiated by the10.6μm laser, the photovoltage depends on the laser spot and undergoes a sign reversal as the laser spot travels from one electrode to another. The changeover in sign occurs at the middle of two electrodes. When the laser spot irradiated nearly on the electrode, the largest photovoltage of ~3 mV with a rise time of several seconds was observed. When the film was illuminated by the1.064μm pulse laser, the peak photovoltage reaches ~2.8 mV and the rise time and full width at half-maximum are ~1.5 ns and ~3 ns, respectively. The present results suggest that the Boron-doped ZnO thin film can be utilized in an infrared sensitive detector at room temperature.