Haibo Yin

Acid Volatile Sulfides and Simultaneously Extracted Metals in a Metal-Polluted Area of Taihu Lake, China

There were similar distribution characteristics for acid volatile sulfides (AVS) and simultaneously extracted metals (SEM) in surface sediments, and the concentrations of AVS and SEM decreased from the deposition area to the center of the bay (lake). The ratio of AVS to SEM was <1 in the surface sediments, indicating that heavy metals in surface sediments may be bioavailable. The concentration of AVS increased with sediment depth, followed by a decrease with large variation, while the concentration of SEM remained constant. By comparing the concentration of SEM with total metals, it was shown that extracted Cu and Ni decreased with sediment depth, indicating increasing association of Cu and Ni with sulfides in deeper sediment layers. The lower extracted ratios for Pb and Zn compared with sulfidic sediment illustrated that AVS should not have strong control on sediment Pb and Zn. The molar ratio of AVS and reactive iron showed that heavy metals were dynamic and active in sediments in this lake.

An investigation on InxGa1−xN/GaN multiple quantum well solar cells

The conversion efficiency of InxGa1−xN/GaN multiple quantum well solar cells is originally investigated in theory based on the ideal diode model and the ideal unity quantum well model. The results reveal that the conversion efficiency partially depends on the width of the quantum well and the thickness of the barrier region but is dominated by the number of quantum wells and indium content of InxGa1−xN. The calculated results are found to be basically trustworthy by comparing with reported experimental results. An In0.15Ga0.85N/GaN multiple quantum well solar cell is successfully fabricated with a conversion efficiency of 0.2%. The main discrepancy between calculated and experimental results is the material quality and manufacturing technology which need to be improved.

InGaN/GaN multiple quantum well solar cells with an enhanced open-circuit voltage

In this paper, InGaN/GaN multiple quantum well solar cells (MQWSCs) with an In content of 0.15 are fabricated and studied. The short-circuit density, fill factor and open-circuit voltage (Voc) of the device are 0.7 mA/cm2, 0.40 and 2.22 V, respectively. The results exhibit a significant enhancement of Voc compared with those of InGaN-based hetero and homojunction cells. This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing Voc of an In-rich InxGa1−xN solar cell. The device exhibits an external quantum efficiency (EQE) of 36% (7%) at 388 nm (430 nm). The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.

Two-dimensional electron and hole gases in InxGa1−xN/AlyGa1−yN/GaN heterostructure for enhancement mode operation

In this paper, a numerical study of In x Ga1 − x N/Al y Ga1 − y N/GaN heterostructure is presented. The dependence of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) sheet densities on variables, such as In x Ga1 − x N layer thickness and In content, and Al y Ga1 − y N barrier layer thickness and Al content, are systematically investigated. The effect of P-type doping in In x Ga1 − x N on 2DEG and 2DHG sheet densities in this heterostructure is also studied. It is shown that the strong reverse electric field in In x Ga1 − x N cap layer contributes to the depletion of 2DEG at the Al y Ga1- y N/GaN interface. When In x Ga1 − x N layer thickness and In content increases, 2DEG sheet density decreases significantly. P-type doping shows less influence on 2DEG compared to the polarization electric field in In x Ga1 − x N layer. In addition, there exist critical values for all the variables beyond which 2DHG appears at the interface of In x Ga1 − x N/Al y Ga1 − y N. Once 2DHG appears, it will prevent 2DEG from being further depleted. With proper design of Al y Ga1 − y N layer, the coexistence of 2DEG and 2DHG in In x Ga1 − x N/Al y Ga1 − y N/GaN structure can be avoided, showing that this structure has great potential in the fabrication of enhancement mode (E-mode) high electron mobility transistors.

Influence of electric field on persistent photoconductivity in unintentionally doped n-type GaN

The influence of electric field on persistent photoconductivity in unintentionally doped n-GaN is investigated. It was found that under higher electric field the build-up course was slowed down while the decay course was accelerated. After a higher-voltage pulse, it was observed that the current dropped to a value lower than the dark current, and a current increase that lasted for thousands of seconds was observed. It is suggested that the above phenomena should be caused by the increase in capture rate of electron traps with electric field and are related to the Coulomb-repulsive characteristic of defects related to persistent photoconductivity.

Tunable density of two-dimensional electron gas in GaN-based heterostructures The effects of buffer acceptor and channel width

This is a theoretical study of GaN-based heterostructures with unintentionally doped (UID) GaN channel layer and high-resistivity (HR) GaN buffer layer doped by deep acceptors. Self-consistent Schrodinger-Poisson (SP) numerical simulation shows that, by increasing the acceptor concentration in the HR buffer or narrowing the width of UID channel, the quantum confinement of two-dimensional electron gas (2DEG) is enhanced, while the sheet density of 2DEG is reduced. The tuning effect of 2DEG density is attributed to the depletion effect of negative space charges composed of ionized acceptors located in the region between the UID channel and the Fermi-level pinned region in the HR buffer. For the heterostructure without the UID channel, the 2DEG can be depleted as the acceptor concentration is beyond a critical value. However, by inserting a UID channel layer, the depletion effect of buffer acceptor on 2DEG density is reduced. To gain a further insight into the physics, a simple analytical model is developed,...

High quality GaN-based LED epitaxial layers grown in a homemade MOCVD system

A homemade 7 × 2 inch MOCVD system is presented. With this system, high quality GaN epitaxial layers, InGaN/GaN multi-quantum wells and blue LED structural epitaxial layers have been successfully grown. The non-uniformity of undoped GaN epitaxial layers is as low as 2.86%. Using the LED structural epitaxial layers, blue LED chips with area of 350 × 350 μm2 were fabricated. Under 20 mA injection current, the optical output power of the blue LED is 8.62 mW.

Comparison of as-grown and annealed GaN/InGaN:Mg samples

Mg-doped InGaN was grown on unintentionally doped GaN layer, and Mg and defect behaviours in both GaN and InGaN?:?Mg were investigated through photoluminescence measurement at 7?K. Mg acceptor was found in unintentionally doped GaN after thermal annealing in N2 ambient, and Mg activation energy was estimated to be 200?meV and 110?meV for GaN and InGaN, respectively. Particularly, the ultraviolet band (3.0?3.2?eV) in the GaN layer was infrequently observed in the unannealed sample but quenched in the annealed sample; this band may be associated with oxygen-substituted nitrogen defects. Moreover, the measurement errors of photoluminescence and x-ray diffraction originated from strain were taken into account.

Impact of dual field plates on drain current degradation in InAlN/AlN/GaN HEMTs

InAlN/AlN/GaN HEMTs with both source and gate dual field-plates (FPs) are proposed. To investigate the influence of dual FPs on the devices characteristics, two types of devices with gate FP and without FPs were fabricated and tested. The devices were subjected to different kinds of short-term direct current bias (DC-bias) stress conditions. The results show that after the off-state bias stress, the drain current reduction rate of the devices with dual FPs was 3.32%, which was less than that in both devices with a gate FP of 7.57% and devices without FPs of 14.63%. The current collapse of the HEMTs with dual FPs was relieved due to the increase of electric field uniformity between the gate and drain. The degradation of the output characteristics was more serious after the on-state bias stress. In addition, the effects of bias stress on the transfer characteristics of the devices were studied, and the trapping processes under different stress conditions in the devices were discussed.

Theoretical analysis of induction heating in high-temperature epitaxial growth system

The temperature uniformity and heating efficiency in a high-temperature epitaxial growth system were investigated by modeling and simulating. The finite element method (FEM) was used to calculate the distribution of magnetic field and temperature field in the reactor of growth system. The simulation results showed that due to the skin effect and heat conduction in the conventional susceptor, the temperature distribution of the wafer on the susceptor is not uniform. However, the temperature uniformity of the wafer can be greatly improved by adding an air gap below the wafer. The existence of air gap effectively reduced the temperature on the center of the wafer, and the effect of its radius on temperature uniformity was studied. By calculating different frequencies and coil currents, the temperature and heating rate in the reactor with modified susceptor were investigated. The results indicated that higher temperature and faster heating rate can be obtained by increasing heating frequency and coil current. However, both lower and higher frequency will bring worse temperature uniformity.