Yan Jianchang

Advances and prospects in nitrides based light-emitting-diodes

Due to their low power consumption, long lifetime and high efficiency, nitrides based white light-emitting-diodes (LEDs) have long been considered to be a promising technology for next generation illumination. In this work, we provide a brief review of the development of GaN based LEDs. Some pioneering and significant experiment results of our group and the overview of the recent progress in this field are presented. We hope it can provide some meaningful information for the development of high efficiency GaN based LEDs and solid-state-lighting.

Characterization of quaternary AlInGaN epilayers and polarization-reduced InGaN/AlInGaN MQW grown by MOCVD

We have demonstrated the growth of quaternary AlInGaN compounds at different growth temperatures and pressures with metalorganic chemical vapor deposition (MOCVD). The optical properties of the samples have been investigated by photoluminescence (PL) at different temperatures. The results show that the sample grown at higher temperature (850 °C) exhibits the best optical quality for its sharp band edge luminescence and weak yellow luminescence. The AlInGaN exhibited three-dimensional (3D) growth mode at higher pressure. The band edge emission almost disappeared. With the optimization of AlInGaN growth parameters, we replaced the traditional barrier in InGaN/GaN multiple quantum wells (MQWs) with AlInGaN barriers. The peak wavelength for the InGaN/AlInGaN-MQW based light emitting diodes (LEDs) was very stable at various injection current levels because of the polarization-matched InGaN/AlInGaN MQWs.

High quality AlGaN grown on a high temperature AIN template by MOCVD

A high temperature AIN template was grown on sapphire substrate by metalorganic chemical vapor deposition. AFM results showed that the root mean square of the surface roughness was just 0.11 nm. Optical transmission spectrum and high resolution X-ray diffraction (XRD) characterization both proved the high quality of the AIN template. The XRD (002) rocking curve full width at half maximum (FWHM) was about 53.7 arcsec and (102) FWHM was about 625 arcsec. The densities of screw threading dislocations (TDs) and edge TDs were estimated to be ~ 6 × 106 cm−2 and ~ 4.7 × 109 cm−2. AlGaN of Al composition 80.2% was further grown on the AIN template. The RMS of the surface roughness was about 0.51 nm. XRD reciprocal space mapping was carried out to accurately determine the Al composition and relaxation status in the AlGaN epilayer. The XRD (002) rocking curve FWHM of the AlGaN epilayer was about 140 arcsec and (102) FWHM was about 537 arcsec. The density of screw TDs was estimated to be ~ 4 × 107 cm−2 and that of edge TDs was ~ 3.3 × 109 cm−2. These values all prove the high quality of the AIN template and AlGaN epilayer.

Localized deep levels in AlxGa1−xN epitaxial films with various Al compositions

By using high-temperature deep-level transient spectroscopy (HT-DLTS) and other electrical measurement techniques, localized deep levels in n-type AlxGa1−xN epitaxial films with various Al compositions (x = 0, 0.14, 0.24, 0.33, and 0.43) have been investigated. It is found that there are three distinct deep levels in AlxGa1−xN films, whose level position with respect to the conduction band increases as Al composition increases. The dominant defect level with the activation energy deeper than 1.0 eV below the conduction band closely follows the Fermi level stabilization energy, indicating that its origin may be related to the defect complex, including the anti-site defects and divacancies in AlxGa1−xN films.

The effect of δ-doping and modulation-doping on Si-doped high Al content n-AlxGa1−xN grown by MOCVD

The effect of periodic delta-doping and modulation-doping on high Al content n-AlxGa1−xN (x = 0.55) epilayers grown by MOCVD has been investigated. Measured by XRD, AFM, contactless sheet resistance, and Hall-effect tests, δ-doped and modulation-doped n-AlxGa1−xN have better crystal quality, surface morphology and electrical properties as compared with uniformly-doped n-AlxGa1−xN. These improvements are attributed to the SiNx growth mask induced by δ-doping layers and the dislocation-blocking effect induced by both growth techniques. In addition, due to the broadened doping profile ascribed to enhanced dopant diffusion at high growth temperatures (1150 °C) of n-Al0.55Ga0.45N, modulation-doped n-Al0.55Ga0.45N has similar properties as δ-doped n-Al0.55Ga0.45N.