Z. L. Xie

Microstructure and dislocation of epitaxial InN films revealed by high resolution x-ray diffraction

This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.

Anisotropic crystallographic properties, strain, and their effects on band structure of m-plane GaN on LiAlO2(100)

The m-plane GaN films grown on LiAlO2(100) by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson–Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN[0001] in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress exx=−0.79% and ezz=−0.14% with an out-of-plane dilatation eyy=0.38%. This anisotropic strain further separates the energy levels of top valence band at Γ point. The energy splitting as 37meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature.

Structural and magnetic properties in Mn-doped GaN grown by metal organic chemical vapor deposition

Mn-doped GaN epitaxial films (Ga1−xMnxN) were grown on sapphire (0001) by metal organic chemical vapor deposition. Mn concentration was determined by energy dispersive spectrometry. For Ga1−xMnxN with x up to 0.027, no secondary phases except for GaN were detected by high resolution x-ray diffractometer. Raman scattering spectra show that the longitudinal optical phonon mode A1(LO) of Ga1−xMnxN shifts toward lower frequency with increasing Mn concentration due to substitutional Mn incorporation. Electron spin resonance (ESR) measurements were performed and highly anisotropic sixfold hyperfine line indicates that the ionized Mn2+ substitutes for Ga3+ ions. However, magnetometry reveals that all homogenous Ga1−xMnxN show paramagneticlike behaviors. From Brillouin function fit and ESR spectra, it is concluded that Mn ions are present as isolated paramagnetic centers.

High-quality Schottky contacts to n-InGaN alloys prepared for photovoltaic devices

Large-area Au∕Pt∕n-In0.2Ga0.8N Schottky contacts have been fabricated for photovoltaic devices. The current transport mechanisms of the Schottky contacts to n-In0.2Ga0.8N with different background carrier concentrations are investigated. The thermionic emission is a dominating current transport mechanism at the Pt∕n-InGaN interface in a low background carrier concentration sample, while the defect-assisted tunneling current and trap-related recombination current play important roles in high background carrier concentration samples. The Schottky diode fabricated using the low background carrier concentration sample gives much better Schottky barrier characteristics and exhibits a three to four order of magnitude higher spectral responsivity and a larger rejection ratio in comparison with those fabricated using the high background carrier concentration samples.

Composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors

We report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/α-Al2O3(0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al0.4Ga0.6N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.

Improved Performances of InGaN Schottky Photodetectors by Inducing a Thin Insulator Layer and Mesa Process

The performances of InGaN Schottky photodetectors with varied fabrication processes were investigated. The photoresponse and dark current of InGaN Schottky photodetectors can be obviously improved by inserting a thin Si3N4 passivation layer between the InGaN layer and the Schottky metal. Furthermore, a mesa process gives not only a further increase in the photoresponse but also a pronounced reduction in the reverse leakage current of about two orders of magnitude. A lateral surface leakage current mechanism associated with the 2-D variable-range hopping conduction through high-density surface states in InGaN is proposed to explain the reduction of the reverse leakage current after etching the mesa.

Biaxial and uniaxial strain effects on the ultraviolet emission efficiencies of AlxGa1−xN films with different Al concentrations

The influences of biaxial and uniaxial strain on the ultraviolet emission efficiencies of both c- and m-plane AlxGa1−xN films with different Al concentrations are investigated under the framework of k⋅p perturbation theory. The optimal high efficiency windows, for ultraviolet light emissions are quantitatively estimated. c-plane AlxGa1−xN modified by uniaxial strain, shows more advantages over biaxial-strained AlxGa1−xN. This is due to the relatively more flexible tuning range and the advantage of obtaining pure linear polarization, which can be utilized to design polarized emission devices. For m-plane AlxGa1−xN, there are always in-plane polarized emissions under both biaxial and uniaxial strain conditions, thus, it is more likely to obtain high surface emission efficiency.

Strain-modulated valence band engineering for enhancement of surface emission in polar and nonpolar plane AlN films

The k⋅p perturbation theory is adopted to calculate the strain-modulated excitonic transition energies and their polarization properties in c- and m-plane AlN. The two topmost valence subbands exchange their band characteristics at the degenerate point where ezz=0.98% and exx=eyy=−1.70%. The surface emission efficiency of c-plane AlN films can be dramatically enhanced with ezz>0.98% (exx=eyy<−1.70%), where the lowest excitonic transition is predominantly z-polarized. Besides, nonpolar plane (m- or a-plane) AlN experiencing anisotropic in-plane strain can be chosen as a candidate for enhancing the surface emission efficiency by proper strain manipulation.

Impact of effective shear strain on the equilibrium phases and polarization states of PbTiO3 thin film

Many ways can be used to tune in-plane strains and thus to tailor the physical properties of ferroelectric films. Impact of effective shear strain on the equilibrium phases and polarization states of single-domain PbTiO3 thin film is investigated by a thermodynamic theory. The modeling results indicate that not only shear strain but also the orientations of polarization components produce profound impacts on the equilibrium phases of thin film, which is essentially different from the cases of S6=0. Different components of polarization can be controlled by temperature, in-plane strains, and especially shear strain.

Impact of lattice strain on the phase formation, polarization, and dielectric constant of PbZr1−xTixO3 films

A phenomenological thermodynamic theory of ferroelectric thin films on noncubic substrates is developed using a nontraditional form of the thermodynamic potential. For single-domain PbZr1−xTixO3 50∕50 film, the “lattice strain-temperature” phase diagram is constructed. It is found that the lattice strain induces a shift of the Curie temperature of the ferroelectric transition. The unusual r phase that is forbidden in single crystals and bulk ceramics appears in thin film. The “lattice strain-polarization” diagram and “lattice strain-dielectric constant” diagram at room temperature are also predicted. They all show discontinuous at the transition interface.