Z. J. Yang

Peking University, Beijing, Beijing Shi, China

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Publications (45)106.8 Total impact

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    ABSTRACT: (1 1 2¯ 0) a-plane gallium nitride has been investigated by Raman spectroscopy and X-ray diffraction. Strains of a-plane GaN grown on LT-GaN nucleation layer were both compressive along [1 1¯ 0 0] and [0 0 0 2] orientations. However, strains of GaN epitaxied on HT-AlN buffer were tensile along [0 0 0 2] and compressive along [1 1¯ 0 0] GaN orientation. The crystalline anisotropy and quality were also improved by the HT-AlN layer. Atomic force microscopy was utilized for analyzing the HT-AlN buffer layer. We ascribed this divergence of strain and improvement of crystalline to the thermal expansion and lattice mismatch effects of HT-AlN buffer during the metalorganic vapor phase epitaxy.
    Applied Surface Science 05/2012; 258(15):5579–5582. · 2.11 Impact Factor
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    ABSTRACT: Defects and strain control in AlxGa1−xN/GaN multiple quantum wells (MQWs) for intersubband transitions (ISBTs) at atmospheric window grown on GaN/sapphire templates by metal-organic chemical vapor deposition have been investigated adopting strain modulation technique using AlN/GaN superlattices (SLs) interlayer. It is found that cracking in the MQWs can be effectively avoided adopting AlN/GaN SLs interlayer. It is demonstrated that AlN/GaN SLs interlayer acts as a flexible layer and relieves most of the tensile strain through buried microcracks in AlN/GaN SLs interlayer. The intersubband absorptions at 3.6–4.1 μm wavelength region have been observed on the crack-free AlxGa1−xN/GaN MQWs. Our results open up prospects to realize crack-free and high quality AlxGa1−xN/GaN MQWs on GaN/sapphire templates for ISBTs devices at 3–5 μm atmospheric window.
    Applied Physics Letters 03/2011; 98(13):132105-132105-3. · 3.79 Impact Factor
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    ABSTRACT: Optically specular a-plane GaN was grown on r-sapphire substrate by metal–organic chemical vapor deposition (MOCVD). Surface morphology and crystal structure anisotropic behavior related to defects with a specific distribution were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). Cathodoluminescence (CL) images and depth-profiling spectra show Basel stacking faults (BSFs) related emission at 3.42 eV, yellow band emission at 2.25 and 3.00 eV emission bands of the a-plane GaN. From the results of CL and transmission electron microscopy (TEM), the origin of the blue emission band was attributed to donor–acceptor pair (DAP) emission correlated with prismatic stacking faults (PSFs).
    Journal of Crystal Growth 03/2011; 318(1):423-426. · 1.55 Impact Factor
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    ABSTRACT: Magnetotransport properties of the two-dimensional electron gas (2DEG) in lattice-matched In0.18Al0.82N/AlN/GaN heterostructures have been studied at low temperatures and high magnetic fields. The double subband occupancy of the 2DEG in the triangular quantum well at the heterointerface is observed. The 2DEG density is determined to be 2.09×1013 cm−2 and the energy separation between the first and the second subbands is 191 meV. Both of them are significantly higher than those in AlxGa1−xN/AlN/GaN heterostructures owing to the stronger spontaneous polarization effect. The evident difference of the quantum scattering times in the two subbands of the 2DEG indicates that the interface roughness scattering plays an important role in the transport properties of the 2DEG in InxAl1−xN/AlN/GaN heterostructures.
    Journal of Applied Physics 01/2011; 109(1):016102-016102-3. · 2.21 Impact Factor
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    ABSTRACT: Light emitting diodes (LEDs) using InGaN/GaN quantum wells (QWs) with thin low temperature GaN (LT-GaN) layers bounding each InGaN layer are grown by metal-organic vapor phase epitaxy. The light output power of such LEDs increases by a factor of 2 at a drive current density of 35 A / cm <sup>2</sup> compared to that from reference LEDs without the LT-GaN. The blueshift in the emission wavelength is 5.2 nm when the current density increases from 3 to 50 A / cm <sup>2</sup> , which is much smaller than the shift 8.1 nm from reference LEDs. Moreover, the efficiency droop at high current injection is also reduced by 28%, and current density at which peak efficiency is observed increases from 1 to 2 A / cm <sup>2</sup> . High resolution transmission electron microscopy of the QWs bounded with LT-GaN shows higher quality and less strain compared to the reference samples. The better performance of LEDs incorporating the LT-GaN layers is attributed to suppressed polarization from piezoelectric fields.
    Journal of Applied Physics 06/2010; · 2.21 Impact Factor
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    ABSTRACT: In <sub> x </sub> Al <sub>1- x </sub> N epilayers ( ∼200 nm thick) under different strain states were grown on GaN templates by metalorganic chemical vapor deposition. When the strain is small (0.166≤ x ≤0.208) , In <sub> x </sub> Al <sub>1- x </sub> N epilayers are almost fully coherent with the GaN templates, and the surface presents similar characteristic of small hillocks and uniform pits. In the case of large tensile strain, cracks emerged on the surface, but the surface morphology is less influenced compared to the samples with small strain. However, with large compressive strain, the surface roughness dramatically increased and additional smaller pits emerged with partial strain relaxation occurring during growth. In addition, the microstructures were further investigated by transmission electron microscopy. It is demonstrated that even slight relaxation of compressive strain can lead to notable influence on the structural quality and surface morphology of In <sub> x </sub> Al <sub>1- x </sub> N films.
    Journal of Applied Physics 03/2010; · 2.21 Impact Factor
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    ABSTRACT: Near-lattice-matched and highly compressive-strained InxAl1−xN epilayers were grown on GaN templates by metalorganic chemical vapor deposition. The V-defects associated with screw-component threading dislocations (TDs) were found in all the InxAl1−xN layers. Their origin and evolution were investigated through near-lattice-matched In0.173Al0.827N layers with different thicknesses. Furthermore, small V-defects not associated with TDs were also found in InxAl1−xN layers with high In composition (x = 0.231). Stacking mismatch boundaries induced by lattice relaxation in InxAl1−xN epilayers under large strain is believed to be another mechanism forming V-defects.
    Applied Physics Letters 12/2009; 95(23):231909-231909-3. · 3.79 Impact Factor
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    ABSTRACT: A kind of inversion domains (IDs) which originated from the tips of threading dislocations were observed in the Al0.65Ga0.35N layer by transmission electron microscopy (TEM). The IDs showed columnar structures with diameters of 10–20 nm. We find that the dislocations evolved into V-shape boundaries then form IDs. By high resolution TEM analysis, the atomic structure at the boundary was proved to be the IDB∗ structure, in which each atom remains fourfold coordinated without the formation of Ga–Ga or N–N bonds.
    Applied Physics Letters 09/2009; 95(11):112106-112106-3. · 3.79 Impact Factor
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    ABSTRACT: 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.
    Applied Physics Letters 04/2009; 94(15):151907-151907-3. · 3.79 Impact Factor
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    ABSTRACT: 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 0.3μ<sub>B</sub>/ 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.
    Journal of Applied Physics 04/2009; · 2.21 Impact Factor
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    ABSTRACT: Micromagnetic simulations of Co domain walls on nanometer crossbars that join two oppositely magnetized parallel legs of “H” shaped patterns are studied. The crossbar domain walls can twist in the plane of the H-pattern, out of the plane, or swirl, forming Néel, Bloch, or vortex structures, respectively, depending on the initial configurations. An energy phase diagram as a function of the crossbar constriction yields the Néel wall as the energetically most favorable, followed by the Bloch wall, which becomes unstable and changes into a vortex-like wall with increasing crossbar size. Most interestingly, the Néel wall width can either shrink or expand depending on the crossbar dimensions. In the case that both the crossbar length and width are small, desirable, ultranarrow domain walls can be obtained. These findings are useful for spintronic device design based on domain wall pinning via nanonotch and domain-wall magnetoresistance approaches.
    Applied Physics Letters 02/2009; 94(6):062514-062514-3. · 3.79 Impact Factor
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    ABSTRACT: The negative capacitance behavior in light-emitting diodes and laser diodes has been observed and characterized by using ac admittance–voltage method. Experimental results proved that the strong negative capacitance behavior is always accompanied by remarkable light emission. We confirmed that the negative capacitance is an effect of the junction instead of other behavior or measurement error. We presented a numerical calculation by solving one dimension continuity equation based on a simple diode model. The results show that the negative capacitance behavior in light-emitting diodes has great relation to injected carriers recombination in the active region of luminescence.
    Solid-State Electronics 01/2009; 53(3):324-328. · 1.48 Impact Factor
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    ABSTRACT: Morphology and microstructure of dislocation etch pits in GaN epilayers etched by molten KOH have been investigated by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). Three types of etch pits ( α , β , and γ ) are observed. The α type etch pit shows an inversed trapezoidal shape, the β one has a triangular shape, and the γ type one has a combination of triangular and trapezoidal shapes. TEM observation shows that α , β , and γ types etch pits originate from screw, edge, and mixed-type threading dislocations (TDs), respectively. For the screw-type TD, it is easily etched along the steps that the dislocation terminates, and consequently, a small Ga-polar plane is formed to prevent further vertical etching. For the edge-type TD, it is easily etched along the dislocation line. Since the mixed-type TDs have both screw and edge components, the γ type etch pit has a combination of α and β type shapes. It is also found that the chemical stabilization of Ga-polar surface plays an important role in the formation of various types of dislocation etch pits.
    Journal of Applied Physics 01/2009; · 2.21 Impact Factor
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    ABSTRACT: 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.
    Applied Physics Letters 12/2008; 93(26):261117-261117-3. · 3.79 Impact Factor
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    ABSTRACT: The effect of thermal annealing of Ni/AlxGa1−xN/GaN structures on electric properties of AlxGa1−xN/GaN heterostructures has been studied by means of temperature-dependent Hall measurements and deep level transient spectroscopy. It is found that the mobility of the two-dimensional electron gas (2DEG) decreases from 1530 to 986 cm2/V s at room temperature (RT) after annealing the Al0.25Ga0.75N/GaN heterostructure with a 10 nm thick Ni cap layer at 600 °C. The density of the 2DEG is also reduced by 2.0×1012 cm−2 at RT after the annealing, and decreases with increasing temperature between 100 and 460 K. It is determined that an acceptorlike deep level with an activation energy of 1.23 eV and apparent capture cross section of 2.8×10−13 cm2 is introduced into the heterostructures. We believe that the acceptorlike deep level is induced by Ni diffusion during the annealing, and it results in the significant degradation of the transport properties of the 2DEG in the heterostructures.
    Applied Physics Letters 10/2008; 93(17):172102-172102-3. · 3.79 Impact Factor
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    ABSTRACT: A method of reducing threading dislocation (TD) density in AlN epilayers grown on sapphire substrate is reported. By introducing an AlN buffer layer grown by a pulsed atomic-layer epitaxy method, TDs in epitaxial AlN films were greatly decreased. From transmission electron microscopic images, a clear subinterface was observed between the buffer layer and the subsequently continuous grown AlN epilayer. In the vicinity of the subinterface, the redirection, annihilation, and termination of TDs were observed. The increase in lateral growth rate accounted for TD redirection and annihilation in the AlN epilayer. Strain variation between the two regions resulted in the termination of TDs owing to the dislocation line energy minimization.
    Applied Physics Letters 09/2008; 93(12):122104-122104-3. · 3.79 Impact Factor
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    ABSTRACT: Spin splitting of the two-dimensional electron gas (2DEG) in AlxGa1-xN/GaN heterostructures has been investigated by means of magnetotransport measurements under the illumination at low temperatures. The beating patterns in the oscillatory magnetoresistance originating from zero-field spin splitting of the 2DEG are observed in this study. It is found that the spin splitting energy decreases after the illumination. It is also found that the illumination decreases the electric field at AlxGa1-xN/GaN heterointerfaces. Based on the experiments, it is suggested that the zero-field spin splitting of the 2DEG in AlxGa1-xN/GaN heterostructures mainly arises from the Rashba effect. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2008; 5(6):2339 - 2341.
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    ABSTRACT: In this work, the evolution of the InGaN layer growth on the ridge shaped GaN was studied. A mass transport model was presented to simulate the epitaxy process of the InGaN layer. The model consisted of two consecutive components, gas-phase diffusion process and surface diffusion process. The mean lifetime of adatoms on epitaxial surface was associated with their reaction rate in this model. An InGaN layer on ridge shaped GaN, including (0002) and {112} facets, was grown by metal organic chemical vapor deposition to confirm the mass transport model. Gradient indium content distribution and inhomogeneous thickness of the InGaN layer were observed. Simulation of the InGaN layer growth process was performed by finite difference method with the mass transport model. By analyzing the results from calculations and experiments, the origins of the InGaN layer characteristics were attributed to the two diffusion components in the growth process. Surface diffusion resulted in the inhomogeneous thickness and gas-phase diffusion chiefly led to the gradient indium content. In addition, the adatoms reaction rate on epitaxial surface determined their mean lifetime as speculated by the analysis. The demonstration of the growth process of InGaN layer offers valuable insight in obtaining high efficiency white light emitting diodes by selective area growth technology.
    Journal of Applied Physics 01/2008; 103(1):014908-014908-4. · 2.21 Impact Factor
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    ABSTRACT: The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.
    Applied Physics Letters 09/2007; 91(12):121114-121114-3. · 3.79 Impact Factor
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    ABSTRACT: Current transport mechanism in Au / Ni / Ga N Schottky diodes has been investigated using current-voltage characterization technique between 27 and 350 ° C . It is found that the ideality factor n of the diode decreases with increasing temperature when the temperature is lower than 230 ° C , and then increases with increasing temperature when the temperature is higher than 230 ° C . The corresponding Schottky barrier height (SBH) increases all through the temperature range. Thermionic-emission model with a Gaussian distribution of SBHs is thought to be responsible for the electrical behavior at temperatures lower than 230 ° C , while the generation-recombination (GR) process takes place in at temperatures above 230 ° C . The effective Richardson constant is determined to be 24.08 A cm <sup>-2</sup> K <sup>-2</sup> , in excellent agreement with the theoretical value. The extrapolated activation energy of the GR process is determined to be 1.157 eV . Based on the cathodoluminescence measurements, it is suggested that the deep level defects inducing yellow luminescence facilitate the GR process of the current transport in the diodes.
    Applied Physics Letters 09/2007; · 3.79 Impact Factor

Publication Stats

61 Citations
234 Downloads
2k Views
106.80 Total Impact Points

Institutions

  • 1997–2012
    • Peking University
      • • State Key Laboratory for Artificial Microstructure and Mesoscopic Physics
      • • School of Physics
      Beijing, Beijing Shi, China
  • 2007–2009
    • Nanjing University
      • Department of Physics
      Nanjing, Jiangsu Sheng, China