Xiaoyang Yang

Study on the formation of dodecagonal pyramid on nitrogen polar GaN surface etched by hot H3PO4

Hot phosphor acid (H3PO4) etching is presented to form a roughened surface with dodecagonal pyramids on laser lift-off N face GaN grown by metalorganic chemical vapor deposition. A detailed analysis of time evolution of surface morphology is described as a function of etching temperature. The activation energy of the H3PO4 etching process is 1.25 eV, indicating the process is reaction-limited scheme. And it is found that the oblique angle between the facets and the base plane increases as the temperature increases. Thermodynamics and kinetics related factors of the formation mechanism of the dodecagonal pyramid are also discussed. The light output power of a vertical injection light-emitting-diode (LED) with proper roughened surface shows about 2.5 fold increase compared with that of LED without roughened surface.

Structural, optical, and magnetic properties of Cu-implanted GaN films

The structural, optical, and magnetic properties of Cu-implanted GaN films have been investigated. No secondary phase was found within the resolution limit of the instrument but the lattice defects such as vacancies were present in the film. Room temperature ferromagnetism was observed with saturation magnetization of 0.3μB/Cu atom. The field-cooled magnetization curves can be well fitted by a Curie-Weiss model and a standard three-dimensional spin-wave model in the low and high temperature ranges, respectively. Our findings indicate that the vacancylike defects should be considered in understanding the observed magnetic properties of the Cu-implanted GaN films.

Luminescent properties in the strain adjusted phosphor-free GaN based white light-emitting diode

A kind of phosphor-free GaN based white light-emitting diode was fabricated with a strain adjusting InGaN interlayer. The origin of the strain adjusted white luminescent properties was studied with cathodoluminescence, asymmetrically reciprocal space mapping with high resolution x-ray diffraction, and scanning electron microscopy. The yellow and blue components of the electroluminescence spectrum were attributed to the high indium core and the adjacent indium depleted region in the inverted pyramidal pits on the device surface, respectively. These pits existed at the end of the dislocations induced by the strain relaxation process of the InGaN interlayer.

Effects of nitrogen vacancies induced by Mn doping in (Ga,Mn)N films grown by MOCVD

We have investigated the effects of nitrogen vacancies (VN) induced by Mn doping on the electronic structure and transport properties of (Ga,Mn)N films grown by metal organic chemical vapour deposition (MOCVD). The significant increase in n-type carrier concentration in the (Ga,Mn)N film is attributed to the additional VN induced by Mn doping. Temperature-dependent Hall data indicate that the additional VN is the dominant scattering mechanism in the (Ga,Mn)N film in higher temperature regions. The Mn L2,3 x-ray absorption spectra of the (Ga,Mn)N film shows a multiplet structure, indicating that the Mn ions are present mainly in the Mn2+(d5) states. These can be attributed to the electrons transferring from VN, which is well consistent with the electrical properties. An energy level model involving the charge transfer is proposed to explain the observed electronic structure and transport properties in the system.

Novel p-Type Conductive Semiconductor Nanocrystalline Film as the Back Electrode for High-Performance Thin Film Solar Cells

Thin film solar cells, due to the low cost, high efficiency, long-term stability, and consumer applications, have been widely applied for harvesting green energy. All of these thin film solar cells generally adopt various metal thin films as the back electrode, like Mo, Au, Ni, Ag, Al, graphite, and so forth. When they contact with p-type layer, it always produces a Schottky contact with a high contact potential barrier, which greatly affects the cell performance. In this work, we report for the first time to find an appropriate p-type conductive semiconductor film, digenite Cu9S5 nanocrystalline film, as the back electrode for CdTe solar cells as the model device. Its low sheet resistance (16.6 Ω/sq) could compare to that of the commercial TCO films (6-30 Ω/sq), like FTO, ITO, and AZO. Different from the traditonal metal back electrode, it produces a successive gradient-doping region by the controllable Cu diffusion, which greatly reduces the contact potential barrier. Remarkably, it achieved a comparable power conversion efficiency (PCE, 11.3%) with the traditional metal back electrode (Cu/Au thin films, 11.4%) in CdTe cells and a higher PCE (13.8%) with the help of the Au assistant film. We believe it could also act as the back electrode for other thin film solar cells (α-Si, CuInS2, CIGSe, CZTS, etc.), for their performance improvement.

Rectification and tunneling effects enabled by Al2O3 atomic layer deposited on back contact of CdTe solar cells

Atomic layer deposition (ALD) of Aluminum oxide (Al2O3) is employed to optimize the back contact of thin film CdTe solar cells. Al2O3 layers with a thickness of 0.5 nm to 5 nm are tested, and an improved efficiency, up to 12.1%, is found with the 1 nm Al2O3 deposition, compared with the efficiency of 10.7% without Al2O3 modification. The performance improvement stems from the surface modification that optimizes the rectification and tunneling of back contact. The current-voltage analysis indicates that the back contact with 1 nm Al2O3 maintains large tunneling leakage current and improves the filled factor of CdTe cells through the rectification effect. XPS and capacitance-voltage electrical measurement analysis show that the ALD-Al2O3 modification layer features a desired low-density of interface state of 8 × 1010 cm−2 by estimation.

Positron annihilation in (Ga, Mn)N: A study of vacancy-type defects

The vacancy-type defects in (Ga,Mn)N films grown by metal organic chemical vapor deposition were studied by positron annihilation technique. Doppler broadening spectra were measured for the films. Compared to the undoped GaN film, the positron trapping defects in the (Ga,Mn)N films have been changed to a new type defects and its concentration increases with the increasing Mn concentration. By analyzing the S-W correlation plots and our previous results, we identify this type defects in the (Ga,Mn)N as VN-MnGa complex. This type of defects should be considered when understand the magnetic properties in a real (Ga,Mn)N system.

Hybrid n-type Sn1−xTaxO2 nanowalls bonded with graphene-like layers as high performance electrocatalysts for flexible energy conversion devices

We report here hybrid n-type Ta-doped SnO2 (Sn1−xTaxO2) nanowalls (as an electron-rich donor) bonded with graphene-like layers (Sn1−xTaxO2/C) as high performance electrocatalysts for flexible energy conversion devices. SnO2 possesses high electron mobility (125–250 cm2 V−1 S−1), and Ta doping is adopted to increase the electron concentration to further improve the conductivity of the SnO2 film to allow its use as a catalyst support. Our first-principles calculations reveal that the increased electrical conductance is mainly attributed to the increased intrinsic doping effect caused by the substitution of Sn by Ta. The Ta-doped SnO2 not only acts a well conductive support for the close coated graphene-like carbon layers but also pushes electrons to the carbon electrocatalyst to enhance its catalytic performance. Advanced features of these nanowall films include not only a high specific surface area, and good adhesion to substrates, but also flexibility. One application as a counter electrode in fully flexible dye-sensitized solar cells (DSSCs) shows that the optimal power conversion efficiency (PCE) of fully flexible DSSCs is 8.38% under AM1.5G illumination (100 mW cm−2), which is one of the highest PCEs for fully flexible DSSCs.

Structural, optical and magnetic properties of Ga1−xMnxN films grown by MOCVD

In this paper, we have investigated the structural, optical and magnetic properties of Ga1−xMnxN films grown by MOCVD. The valence band structure was analysed by the x-ray photoelectron spectroscopy (XPS) measurements. A different Fermi level shift behaviour was observed with increasing Mn composition in the GaN. This shift behaviour was attributed to the donor-like defects induced by the Mn doping in the heavily doped samples. The presence of these defects was further confirmed by the calthodoluminescence spectra. The additional peak around 2.0 eV in the heavily doped samples corresponds to the intra-d-shell transitions of Mn2+. This Mn2+ state was formed by trapping the electrons released from the donor-like defects, which is quite consistent with the XPS results. Finally, the magnetic properties were also discussed in combination with the above results.

Tuning nanosheet Fe2O3 photoanodes with C3N4 and p-type CoOx decoration for efficient and stable water splitting

Fe2O3 photoanodes are ideal candidates for photoelectrochemical (PEC) water splitting. However, the charge recombination in the bulk and at the photoanode/electrolyte interface decreases their PEC performance. Here, C3N4 and p-type CoOx are firstly decorated on Fe2O3 nanosheets for PEC performance enhancement and mechanism study. The photocurrent densities of Fe2O3/C3N4 and Fe2O3/C3N4/CoOx photoanodes are about 1.6 and 2 times at 1.23 V vs. RHE (reversible hydrogen electrode) compared with that of the Fe2O3 film (0.74 mA cm−2) under simulated sun light irradiation. Correspondingly, their photocurrent onset potentials are negatively shifted by about 0.09 and 0.19 V compared with that of Fe2O3 (0.81 VRHE). The solar-to-hydrogen conversion efficiency reaches 0.17% and the incident photo-to-current conversion efficiency (IPCE) achieves 81.7% at 385 nm for the Fe2O3/C3N4/CoOx hybrid photoanode. The matched band alignments between Fe2O3 and C3N4 result in more efficient charge separation, and the p-type CoOx cocatalyst reduces surface recombination and shows quicker water oxidation reaction kinetics at the semiconductor/electrolyte interface.