Z. L. Miao

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...

Different origins of the yellow luminescence in as-grown high-resistance GaN and unintentional-doped GaN films

The yellow luminescence (YL) in as-grown high-resistance (HR) and unintentional-doped (UID) GaN films grown by metal organic chemical vapor deposition has been investigated by means of photoluminescence and monoenergetic positron annihilation spectroscopy. It is found there is stronger YL in UID-GaN with higher concentration of gallium vacancy (VGa), suggesting that VGa-involved defects are the origin responsible for the YL in UID-GaN. Contrastly, there is much stronger YL in HR-GaN that is nearly free from VGa, suggesting that there is another origin for the YL in HR-GaN, which is thought as the carbon-involved defects. Furthermore, it is found that the HR-GaN film with shorter positron diffusion length Ld exhibits stronger YL. It is suggested that the increased wave function overlap of electrons and holes induced by the extremely strong space localization effect of holes deduced from the short Ld is the vital factor to enhance the YL efficiency in HR-GaN.

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.

Magnetotransport properties of lattice-matched In0.18Al0.82N/AlN/GaN heterostructures

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.

Study of the leakage current mechanism in Schottky contacts to Al0.25Ga0.75N/GaN heterostructures with AlN interlayers

The leakage current mechanism in Schottky contacts (SCs) to Al0.25Ga0.75N/GaN heterostructures incorporated by a thin high-temperature (HT) AlN interlayer has been investigated using current–voltage measurements, atomic force microscopy and deep level transient spectroscopy. It is found that the HT AlN interlayer thickness has a significant effect on the leakage current in SCs. The leakage current density decreases to 1.1 × 10−4 A cm−2 when the growth time of the AlN interlayer increases from 0 to 10 s, and then changes to increase with increasing growth time. Correspondingly, the heterostructure with the AlN growth time of 10 s has the least number of surface pinholes. The thickness of the HT AlN also influences the density of electron traps with the activation energy of 0.762 eV in an Al0.25Ga0.75N barrier. It is suggested that the HT AlN interlayer adjusts the microstructure and the defect state density in the Al1−xGaxN barrier, and the leakage via these defect states makes the main contribution to the leakage current in SCs to Al1−xGaxN/GaN heterostructures.

Influence of ultrathin AlN interlayer on the microstructure and the electrical transport properties of AlxGa1−xN/GaN heterostructures

The microstructure and electrical properties of Al0.25Ga0.75N/GaN heterostructures with various AlN thicknesses have been investigated. An optimum thickness of AlN interlayer can remarkably improve the microstructure of Al0.25Ga0.75N barrier with the most uniform strain and the lowest density of threading dislocations, leading to the highest Hall mobility of the two-dimensional electron gas in the heterostructures. Transmission electron microscopy images show that the AlN interlayer with an optimum thickness can make the threading dislocations bend and be annihilated with each other in the vicinity of the heterointerface due to the larger mismatch strain between AlN interlayer and GaN. We believe that such behavior reduces the local strain and improves the uniformity of the strain in the AlxGa1−xN barrier, and thus depresses the scattering induced by the fluctuations of the piezoelectric interface charge owing to the nonuniformity of piezoelectric polarization field in AlxGa1−xN/GaN heterostructures.

Mechanical properties of AlxGa1−xN films with high Al composition grown on AlN/sapphire templates

Mechanical properties of AlxGa1−xN thin films with high Al composition (0.33⩽x⩽1) grown on AlN/sapphire templates have been investigated by means of the nanoindentation technique. It is found that Young’s modulus E of the films increases with increasing Al composition. In addition, it is also found that the occurrence of the clear and sudden displacement discontinuity (“pop-in”) in the plastic deformation (PD) process is dependent on Al composition in AlxGa1−xN films. The higher Al composition results in less occurrence of the pop-in in the PD process of the films. With increasing Al composition, it is believed that the increase of the bond strength and the decrease of the lattice mismatch between AlxGa1−xN films and AlN/sapphire templates result in greater resistance to the formation of dislocations, which is responsible for the pop-in behavior in AlxGa1−xN films.

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.

Ni diffusion and its influence on electrical properties of AlxGa1−xN∕GaN heterostructures

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 1530to986cm2∕Vs at room temperature (RT) after annealing the Al0.25Ga0.75N∕GaN heterostructure with a 10nm thick Ni cap layer at 600°C. The density of the 2DEG is also reduced by 2.0×1012cm−2 at RT after the annealing, and decreases with increasing temperature between 100 and 460K. It is determined that an acceptorlike deep level with an activation energy of 1.23eV and apparent capture cross section of 2.8×10−13cm2 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.