Xiufang Zhang

Microstructure and origin of dislocation etch pits in GaN epilayers grown by metal organic chemical vapor deposition

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 surfac...

Reduction in threading dislocation densities in AlN epilayer by introducing a pulsed atomic-layer epitaxial buffer layer

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.

The origin and evolution of V-defects in InxAl1−xN epilayers grown by metalorganic chemical vapor deposition

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.

Strain effects on InxAl1−xN crystalline quality grown on GaN templates by metalorganic chemical vapor deposition

InxAl1−xN 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), InxAl1−xN 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 InxAl1−xN films.

Morphology of threading dislocations in high-resistivity GaN films observed by transmission electron microscopy

Morphology of threading dislocations (TDs) in GaN films with different resistivities grown on sapphire by means of metal organic chemical vapor deposition has been investigated using transmission electron microscopy (TEM). GaN films with different resistivities are achieved at various annealing pressures of the nucleation layer (NL). It is observed that the TDs are almost all straight and perpendicular to the sapphire surface in high-resistivity GaN films, while they are significantly bent and interactive in low-resistivity GaN films. The analysis results based on the x-ray diffractometry and TEM demonstrate that the density and morphology of TDs change with annealing pressure of the NL. It is concluded that the annealing pressure of the NL effectively controls the size, density, and coalescence rate of the islands, and thus determines the density and morphology of TDs in GaN.

A physical based model to predict performance degradation of FinFET accounting for interface state distribution effect due to hot carrier injection

Abstract A physical based model for predicting the performance degradation of the FinFET is developed accounting for the interface state distribution effect due to hot carrier injection (HCI). The non-uniform distribution of interface state along the FinFET channel is first extracted by a forward gated-diode method and then reproduced by an empirical model. From this, a physical-based device model, which accounts for the interface state distribution effect, is developed to predict the performance degradation of FinFET. The result shows that the developed model not only matches well with the experimental data of FinFET in all operation regions, but also predicts the asymmetric degradation of saturation drain current in forward and reverse operation mode. Finally, the impact of HCI to a 6-T SRAM cell is simulated using HSPICE.

Transmission electron microscopy investigation of inversion domain boundary in Al0.65Ga0.35N grown on AlN/sapphire template

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.

Morphology and microstructure evolution of AlxGa1−xN epilayers grown on GaN/sapphire templates with AlN interlayers observed by transmission electron microscopy

Morphology and microstructure evolution of Al0.3Ga0.7N epilayers grown on GaN/sapphire templates with low-temperature (LT) AlN interlayers (ILs) by means of metal organic chemical vapor deposition have been investigated by transmission electron microscopy and atomic force microscopy. It is found that the IL improves the surface morphology, and suppresses edge-type threading dislocations (TDs). When the IL thickness is 20 nm, there is the lowest density of the edge-type TD with 8.7×108 cm-2. But the edge-type TD density increases somewhat as IL thickness increases to 40 nm. It is believed that two mechanisms determine the microstructure evolution of the AlxGa1-xN epilayers. One is the TDs suppression effect of LT-AlN ILs that ILs can provide an interface for edge-type TDs termination. Another is the TDs introduction effect of ILs that new edge-type TDs are produced. Due to the lattice mismatch between AlN, GaN and AlxGa1-xN, the strain in AlxGa1-xN epilayers is modified by inserting the AlN IL, and thus changes the formation of the edge-type TDs.

Asymmetric issues of FinFET device after hot carrier injection and impact on digital and analog circuits

This paper presents the asymmetric issue of FinFET device after hot carrier injection (HCI) effect and impact on the digital and analog circuits. The interface state distribution along the FinFET channel is first extracted from hot carrier injection experimental data, and then develops a compact FinFET model to simulate the impact on asymmetric distribution of interface states to the device characteristics. The results show that the asymmetric degradation is much more significant in Ids-Vds characteristics than in Ids-Vgs characteristics. On the other hand, digital and analogy circuits exhibit different asymmetric performance degradation in various operation cases.

Microstructure of epitaxial superconductive YBa2Cu3O7 thin films prepared at different deposition rates

A study is reported on the growth mechanism of YBa2Cu3O7 with different growth speeds by high resolution transmission microscopy (HRTEM) and analysis of the interface and thin film microstructure. Two thin films were synthesized by pulse laser deposition on [100], miscut 5°, SrTiO3 substrate at 820 °C, one with a pulse laser frequency of 1 Hz and one with 6 Hz. Cross-sections were studied by an H-9000 NAR HRTEM along the [010] direction. The growth process of the sample made at 1 Hz was as follows. First, distorted step flow growth occurred on a step-mediated substrate surface of 3–4 cells thickness. Second, about a 15 nm thickness of island shape growth becomes superimposed on the area of the step flow layer. Finally, thin film growth occurred but with growth fluctuation. The sample made at 6 Hz showed the characteristics of island growth; the growth area of island or ball shape was of small size and dense distribution, and seemed to be a confused mosaic stack. The influence of growth speed on YBCO epitaxial film microstructure was studied explicitly by HRTEM.