Xinzhong Wang

Cubic-phase GaN light-emitting diodes

The feasibility of growing device-quality cubic GaN/GaAs(001) films by metal organic chemical vapor deposition has been demonstrated. The optical quality of the GaN films was characterized by room-temperature photoluminescence measurements, which shows a full width at half maximum of 46 meV. The structural quality of the films was investigated by transmission electron microscopy. There are submicron-size grains free from threading dislocations and stacking faults. More importantly, a cubic-phase GaN blue light-emitting diode has been fabricated. The device process, which is very simple and compatible with current GaAs technology, indicates a promising future for the blue light-emitting diode

Improved GaN film overgrown with a molybdenum nanoisland mask

We report the improved crystalline and optical quality of GaN film overgrown by hydride vapor phase epitaxy adopting a molybdenum (Mo) nanoisland mask (MNM). The MNM is fabricated following thermal annealing of the nanometer-thick Mo film deposited by electron-beam evaporation. The full width at half maximum values of high-resolution x-ray diffraction (HRXRD) rocking curves for the GaN film with MNM are 188arcsec (002 reflection) and 219arcsec (102 reflection), while those for the GaN film without MNM are 256 and 364arcsec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with MNM. Photoluminescence spectra measurements reveal the compressive strain relaxation and improvement in the quality of the overgrown GaN film with MNM as compared to the regrown GaN film without MNM, which is consistent with the trend observed by HRXRD.

Thick GaN grown on a nanoporous GaN template by hydride vapor phase epitaxy

High-quality thick gallium nitride (GaN) films were overgrown by hydride vapor phase epitaxy (HVPE) on a nanoporous GaN template which was prepared by inductively coupled plasma etching employing an anodized aluminum oxide mask. An obvious reduction of the dislocation density in the thick GaN layer was demonstrated by high-resolution X-ray diffraction, which exhibited the improved crystalline quality in GaN films overgrown on a nanopattemed surface. Moreover, the peak redshift in photoluminescence and micro-Raman spectroscopy indicates a significant strain relaxation in the HVPE-GaN layer. Compared with conventional overgrowth, such a deposition pathway is a more promising technique for the growth of thick GaN layers.

Low-temperature growth of cubic GaN by metalorganic chemical-vapor deposition

Low-temperature growth of cubic GaN at 520 degrees C was achieved using CCl4 as an additive by metalorganic chemical-vapor deposition (MOCVD) on GaAs substrate. X-Ray measurement confirmed that the films are single-phase cubic GaN. Scanning electron microscopy (SEM) and reflection high-energy electron diffraction (RHEED) were also used to analyze the surface morphology and the quality of films. The evolution of surface morphology suggests that CCl4 can reduce the hopping barrier and thus Ga adatoms are able to diffuse easily on the GaN surface

Research on fabrication and properties of nanoporous GaN epilayers

Gallium nitride (GaN) epilayers with nanopore arrays were fabricated by inductive coupled plasma (ICP) etching using anodic aluminum oxide (AAO) as mask. Nanoporous AAO templates were formed by anodizing the Al films deposited on GaN epilayers. The diameter of the perforations in the AAO masks could be easily controlled by tuning the technique parameters of AAO fabrication process. Cl2/Ar and Cl2/He were employed as etching gas. Scanning electron microscopy (SEM) analysis shows that vertical nanoporous arrays with uniform distribution can directly be transferred from AAO masks to GaN films in some proper conditions. Photoluminescence (PL) spectra, X-ray diffraction (XRD) and Raman spectroscopy were applied to assess properties of the nanoporous GaN films with different average pore diameters and interpore distances.

Effect of AlN intermediate layer on growing GaN film by hydride vapor phase epitaxy

Abstract Thick GaN layer deposited by hydride vapor phase epitaxy (HVPE) on a metalorganic chemical vapor deposition (MOCVD) GaN template with a thin low temperature (LT) AIN intermediate layer was investigated. High resolution X-ray resolution diffraction (HRXRD) shows that the crystalline quality of thick GaN layer was improved compared with the template. As confirmed by atomic force microscopy (AFM) observations, the surface morphology of AIN intermediate layer helps to improve the nucleation of GaN epilayer. Photoluminescence (PL) spectra measurement shows its high optical quality and low compressive stress, and micro Raman measurement confirms the latter result. Thus, the deposition of the LT-AIN interlayer has promoted the growth of an HVPE-GaN layer with an excellent crystalline quality.

Growth of high-quality thick GaN using a tungsten interlayer

A high-quality GaN film was (W-GaN) grown by hydride vapor phase epitaxy (HVPE) on metalorganic chemical vapor deposition (MOCVD) GaN templates with a tungsten (W) interlayer. A sample without interlayer was also grown at the same time for comparison. Significant reductions of dislocation density in W-GaN film is confirmed by the result of high-resolution X-ray diffraction and transmission electron microscope (TEM) observation. The improvement of optical properties of the W-GaN is confirmed by photoluminescence (PL) result. A shift of PL peak suggests that the strain is lower in the W-GaN than the film without W interlayer. This technique offers a potential path to obtain high-quality GaN film as free-standing substrate.

High-Quality Thick GaN Overgrown on an Array of SiO2 Nanomasks by HVPE

We report nanoepitaxy of thick GaN films by hydride vapor-phase epitaxy (HVPE) using a uniform array of SiO 2 nanomasks that were prepared by electron-beam evaporation and anodic aluminum oxide membranes. The controllable size and density of the SiO 2 nanoisland mask were about 65 nm and 10 10 cm -2 , respectively. Subsequent overgrowth takes place selectively on the exposed GaN surface without the SiO 2 masks to the formation of the continuous layer. High-resolution X-ray diffraction shows reduction of threading dislocations in the overgrown layers. Improved optical property and significant stress relaxation in the epilayer are demonstrated by photoluminescence and micro-Raman spectra. The deposition method is a more promising technique for obtaining freestanding GaN substrates with high-quality and uniform features.