Shude Yao

Optical, structural, and nuclear scientific studies of AlGaN with high Al composition

AlGaN epilayers with higher Al-compositions were grown by Metalorganic Chemical Vapor Deposition (MOCVD) on (0001) sapphire. Trimethylgallium (TMGa), trimethylaluminium (TMAl) and NH3 were used as the source precursors for Ga, Al, and N, respectively. A 25 nm AlN nucleation layer was first grown at low-temperature of 590 °C at 300 Torr. Followed, AlxGa1-xN layers were grown at 1080 °C on low-temperature AlN nucleation layers. The heterostructures were characterized by a series of techniques, including x-ray diffraction (XRD), Rutherford backscattering (RBS), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering. Precise Al compositions were determined through XRD, RBS, and SEM combined measurements. Room Temperature Raman Scattering spectra shows three major bands from AlGaN alloys, which are AlN-like, A1 longitudinal optical (LO) phonon modes, and E2 transverse optical (TO) band, respectively, plus several peak comes from the substrate. Raman spectral line shape analysis lead to an optical determination of the electrical property free carrier concentration of AlGaN. The optical properties of AlGaN with high Al composition were presented here.

Rutherford backscattering and optical studies for ZnO thin films on sapphire substrates grown by metalorganic chemical vapor deposition

A series of ZnO thin films with different thicknesses grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) have been studied by different characterization techniques. The optical properties are investigated by photoluminescence (PL), optical transmission (OT) and 1st order derivatives, various angle scanning ellipsometry (VASE). Rutherford Backscattering (RBS) shows the atomic Zn:O ratios with a few percentage aviation from 1:1, and thicknesses in range of 10~230 nm, roughness layer with 10~30nm, which are corresponding to results from atomic force microscopy (AFM), and scanning electron microscopy (SEM). The optical and structure characterization measurements have confirmed the good quality of these epitaxial ZnO materials.

Defects study of MOCVD grown β-Ga2O3 films

Highly (-201) oriented β-Ga2O3 films prepared by metal-organic chemical vapor deposition on (0001) sapphire substrates, undergone different post annealing temperatures to study their resistivity under harsh environment. Both of Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy (TEM) results are exposing a harmony between oxygen vacancies and gallium interstitials. TEM characterization of samples determines a relationship between interstitials and formation of screw dislocations. Cathodoluminecsnece investigated under different applied voltages is found to be applicable to study chemistry of the bulk and surface of β-Ga2O3.

Nuclear science and optical studies of InAlGaP materials grown on GaAs by metalorganic chemical vapor deposition

Quaternary III-V compound InAlGaP, especially In0.5(AlxGa1-x)0.5P which is lattice matched with GaAs, are important materials for visible red-green light emitting diode (LED) and laser diode (LD), solar cell and other optoelectronic and electronic device applications. A set of In0.5(AlxGa1-x)0.5P thin films on GaAs substrates with a wide range of x up to ~80%, were grown by low pressure metalorganic chemical vapor deposition (MOCVD) and studied by a variety of nuclear science and optical analytical techniques, including Rutherford Backscattering Spectrometry (RBS), Raman scattering, photoluminescence (PL), Photoreflectance (PR) and FTIR. Temperature dependent PL-PR measurements over 10-300 K presented the band gap of these InAlGaP materials and variations with composition x and temperature (T). RBS was used to measure the microstructure of AlInGaP films, and through simulation, determine the film thickness and composition precisely. RBS measurement and simulation results indicate a quite fuzzy in the two interfaces, i.e. that there exists diffusion in the majority samples, especially between the AlInGaP layer and substrate. For a certain number of incoming He+ ions,we have proposed a way to determine the error bar by RBS successful. For this series of samples, the error bar of content is around ±1.5%. The error bar of thickness is around ±5.0nm. Different InAlGaP films with different composition and thickness may present different error bars. The results illuminate that RBS is a precise tool to analysis the microstructure of quaternary semiconductor AlInGaP/GaAs samples.

Material properties of MOCVD-grown AlGaN layers influenced by indium-incorporation

A set of AlGaN epilayers were grown on sapphire (0001) substrate by MOCVD, with intermediate growths of low/high temperature AlN nucleation layers. Variable flow rates of trimethylindium (TMIn), 0, 50 and 500 sccm were introduced during growth. Three AlGaN samples were originally designed with similar Al composition of ~20%. Rutherford backscattering (RBS), RT and 10-300K photoluminescence (PL) were used for analyzing the microstructure of thin films. The Al content was calculated to decrease with increasing the In-flow rate. Main PL bands spread over 310-350 nm with peaks in 320-335 nm. PL (10-300K) exhibited anomalous temperature dependent emission behavior (specifically an S-shaped shift, i.e. red-blue-red shifts) of the AlGaN related PL emission. Carriers transfer between different luminescent centers. Abnormally high activation energy was obtained, which shows that the excitons are not in the free states. Raman Scattering and spectral line shape analysis leaded to an optical determination of the electrical property free carrier concentration of AlGaN. Our results on In-doped AlGaN provide useful information for designing UV-LEDs.