Xiangqian Xiu

Nonpolar m-plane thin film GaN and InGaN∕GaN light-emitting diodes on LiAlO2(100) substrates

The nonpolar m-plane (11¯00) thin film GaN and InGaN∕GaN light-emitting diodes (LEDs) grown by metal-organic chemical vapor deposition on LiAlO2 (100) substrates are reported. The LEDs emit green light with output power of 80μW under a direct current of 20mA for a 400×400μm2 device. The current versus voltage (I-V) characteristic of the diode shows soft rectifying properties caused by defects and impurities in the p-n junction. The electroluminescence peak wavelength dependence on injection current, for currents in excess of 20mA, saturates at 515–516nm. This proves the absence of polarization fields in the active region present in c-plane structures. The light output intensity versus current (L-I) characteristic of the diode exhibits a superlinear relation at low injection current caused by nonradiative centers providing a shunt path and a linear light emission zone at high current level when these centers are saturated.

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.

Modification of the valence band structures of polar and nonpolar plane wurtzite-GaN by anisotropic strain

The influence of anisotropic strain on the valence band structure and related properties, including excitonic transition energies, transition polarization selection rules, band-edge hole effective masses, and exciton reduced effective masses, of polar and nonpolar plane GaN are systematically investigated using the well-known k⋅p Hamiltonian approach. We re-examine the band deformation potentials D3 and D4, and interband hydrostatic deformation potentials a1 and a2, and find that they take the values 9.4, −4.7, −3.0, and −12.4 eV, respectively. In order to correctly interpret the optical properties of GaN, the spin-orbit coupling effect cannot be neglected. Our numerical calculations show that pure linear polarization light emissions and absorptions can be obtained. In addition, the two topmost valence subbands can be effectively separated to reduce the band-edge density of state by manipulating the strain states in GaN epilayers, which is favorable for laser diode design. Furthermore, the band-edge hole ...

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.

Low leakage Schottky rectifiers fabricated on homoepitaxial GaN

The development of a vertical GaN Schottky rectifier on freestanding bulk GaN substrate by employing homoepitaxial growth technique is reported. The n−∕n+ Schottky diode structure was grown on conductive GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the homoepitaxial layer determined by cathodoluminescence mapping technique is ∼6×106cm−2 with surface roughness less than 1nm. The fabricated Schottky rectifiers with dielectric overlap edge termination show a low reverse leakage current of <3×10−8A at −100V, which can be explained by the low dislocation density in the epilayer and high Schottky barrier height (1.37eV) of the Pt∕GaN metal-semiconductor contact. Due to the reduced series resistance, the bulk GaN Schottky rectifiers also exhibit a low turn-on voltage of ∼1.3V and an on-resistance of ∼5.3mΩcm2.

Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN

Using the Debye model and existing experimental data of the pyroelectric coefficient of AlN, the temperature dependence of the pyroelectric coefficient as well as the spontaneous polarization of AlN is calculated over a wide temperature range from 0to1000K. The pyroelectric coefficient is proportional to T3 at low temperature and increases acutely from 0 to around 400K, and then increases gently from 400to1000K. It makes AlN uniquely suitable for application in high temperature pyroelectric sensors. The spontaneous polarization of AlN changes a little from 0to1000K, which indicates that the features of III-nitrides based devices will hardly be degraded by the change of the spontaneous polarization.

Preparation of large area freestanding GaN by laser lift-off technology

Abstract Gallium nitride films grown on sapphire substrates were successfully separated by laser radiation. The absorption of the 248-nm radiation by the GaN at the interface results in rapid thermal decomposition of the interfacial layer, yielding metallic Ga and N2 gas. The substrate can be easily removed by heating above the Ga melting point of 29 °C. Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements of GaN before and after lift-off process have been performed, which demonstrated that the separation and transfer process do not alter the structural quality of the GaN films. Further discussions on the threshold energy and crack-free strategies of laser lift-off process have also been presented.

Metal–Semiconductor–Metal Ultraviolet Avalanche Photodiodes Fabricated on Bulk GaN Substrate

We report the first demonstration of a GaN-based planar metal-semiconductor-metal (MSM) ultraviolet (UV) avalanche photodiode (APD). The MSM APD with semitransparent interdigitated Schottky electrodes is fabricated on a low-defect-density GaN homoepitaxial layer grown on a bulk GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the GaN homoepilayer characterized by a cathodoluminescence mapping technique is ~ 5 ×106 cm-2. The photodiode exhibits a low dark current density of ~ 1.4 × 10-9 A/cm2 and a high UV-to-visible rejection ratio up to five orders of magnitude under 20-V bias. At high bias, a room-temperature avalanche gain of more than 1100 is achieved under 365-nm UV illumination. The breakdown voltage of the APD shows a positive temperature coefficient of 0.15 V/K, confirming that the high-voltage gain is dominated by the avalanche breakdown mechanism.

A thermodynamic model and estimation of the experimental value of spontaneous polarization in a wurtzite GaN

Determining the spontaneous polarization is a fundamental problem in the III-nitride field. However the experimental value of the spontaneous polarization has not yet been reported. In this study, a thermodynamic model is proposed to investigate the spontaneous polarization of GaN from the GaN high-pressure phase transition. Total macroscopic polarization is directly expressed as coefficients in expansion, and these coefficients are found to be conducted in experiments. The experimental value of the spontaneous polarization of GaN is estimated to be around −0.022 C/m2.