L.W. Tu

High-dielectric-constant Ta2O5/n-GaN metal-oxide-semiconductor structure

High-dielectric-constant Ta2O5 has been grown on the n-GaN epifilm by rf magnetron sputtering. Photoluminescence measurement has been performed to compare the luminescence intensity with and without the dielectrics. Threefold increase in intensity is obtained, and a surface recombination velocity is estimated to be 3×104 cm/s as an upper limit using a modified dead-layer model. A metal-oxide-semiconductor structure has been fabricated with Al on n-GaN as the ohmic contact and on Ta2O5 as the gate metal. Capacitance-versus-voltage characteristics have been measured. The doping concentration obtained from the depletion regime is compared with the result of Hall measurement, which is 7.0×1016 cm−3. The flat-band voltage is obtained from the high-frequency data, and the effective oxide charge number density is calculated as 4.1×1012 cm−2. Indication of strong inversion appears at low reverse bias due to the high dielectric constant of Ta2O5, and matches closely with calculated values. Hysteresis is observed a...

Hydrogen in InN: A ubiquitous phenomenon in molecular beam epitaxy grown material

We study the unintentional H impurities in relation to the free electron properties of state-of-the-art InN films grown by molecular beam epitaxy (MBE). Enhanced concentrations of H are revealed in the near surface regions of the films, indicating postgrowth surface contamination by H. The near surface hydrogen could not be removed upon thermal annealing and may have significant implications for the surface and bulk free electron properties of InN. The bulk free electron concentrations were found to scale with the bulk H concentrations while no distinct correlation with dislocation density could be inferred, indicating a major role of hydrogen for the unintentional conductivity in MBE InN.

p-GaN/InGaN/n-GaN pedestal nanorods: Effect of postgrowth annealing on the electrical performance

Pedestal p-GaN/InGaN/n-GaN nanorods have been fabricated on n-type Si (111) substrates by properly reducing the growth temperature of the p-GaN surface layer. Continuous p-GaN layers were formed on the top region by accelerated lateral growth, while keeping the underlying nanostructures and physical properties of InGaN and n-GaN intact, making it feasible for large-scale vertical integration. Growth of the p-GaN layer at 500 °C followed by annealing at 600–800 °C improved crystal structures and the overall electrical and luminescence properties of pedestal nanorods.

Effect of temperature and V/III ratio on the initial growth of indium nitride using plasma-assisted metal-organic chemical vapor deposition

The growth of Indium nitride (InN) was studied in the nucleation stage by metal-organic chemical vapor deposition technique using atomic nitrogen from an RF microwave plasma source. Deposition was carried out through a range of substrate temperatures from 375 to 550 °C and at varying V/III ratios from 950 to 3150. We found that the diffusion lifetime of In atoms on the substrate becomes maximized at the growth temperature 475 °C, in which low temperature photoluminescence exhibits the excellent optical properties of the materials with a bandgap of 0.69 eV and a width of 34 meV. In addition, we observed that nitrogen cracking efficiency is significantly improved by using plasma so that high quality InN crystallites were grown with a very low V/III ratio around 950.

Effects of plasma power on material and optical quality of GaN nanorods grown by plasma-assisted molecular beam epitaxy

The growth of wurtzite GaN(0001) nanorods on Si(111) substrates in the nitrogen-rich regime was studied in comparison with nitrogen species generated from a remote inductively coupled plasma source. A reduction in incorporation rate and a substantial deterioration in the quality of GaN nanorods were found to be correlated with an increase in plasma power. On the basis of the observation of defect formation in the initial growth stage and its propagation during the growth, the degradation of material perfection by the increased energy delivered by active nitrogen species at high power was discussed.

Hydrogen In Group‐III Nitrides: An Ion Beam Analysis Study

The doping mechanisms of InN, a promising material for novel optoelectronic and electronic devices, are still not well understood. Unintentional hydrogen doping is one possibility that could explain the unintentional n‐type conductivity in high‐quality nominally undoped InN films. We measured a series of state‐of‐the‐art InN samples grown by molecular beam epitaxy with 2 MeV 4He‐ERDA and RBS, showing the presence of relatively high amounts of hydrogen not only at the surface, but also in a deeper layer. Strong depletion of hydrogen due to the analysing beam was observed and taken into account in the analysis. Here, we report on the details of the analysis and show how the results correlate with the free‐electron concentrations of the samples.

Controlling Surface Morphology and Circumventing Secondary Phase Formation in Non-polar m-GaN by Tuning Nitrogen Activity

For the development of non-polar nitrides based optoelectronic devices, high-quality films with smooth surfaces, free of defects or clusters, are critical. In this work, the mechanisms governing the topography and single phase epitaxy of non-polar m-plane gallium nitride (m-GaN) thin films are studied. The samples were grown using plasma-assisted molecular beam epitaxy on m-plane sapphire substrates. Growth of pure m-GaN thin films, concomitant with smooth surfaces is possible at low radio frequency powers and high growth temperatures as judged by the high resolution x-ray diffraction, field emission scanning electron microscopy, and atomic force microscopy measurements. Defect types and densities are quantified using transmission electron microscopy, while Raman spectroscopy was used to analyze the in-plane stress in the thin films which matches the lattice mismatch analysis. Energy dispersive spectroscopy and cathodoluminescence support a congruent growth and a dominant near band edge emission. From the analysis, a narrow growth window is discovered wherein epitaxial growth of pure m-plane GaN samples free of secondary phases with narrow rocking curves and considerable smooth surfaces are successfully demonstrated.

Growth and characterization of III-nitride based multiple quantum wells for photovoltaic devices

Efficient conversion of solar energy into electricity is crucial to the use of renewable energy. Among the various semiconductors being investigated for photovoltaic conversion, III- nitrides are fervently pursued because of their band gap tenability from 0.65 eV to 3.4 eV by adjusting the indium concentration of InXGa1-XN alloys. This enables the coverage of optical absorption over a wide range of the solar spectrum, thus providing a path to boosting the conversion efficiency. This presentation reports on multiple quantum well (MQW) based solar cells fabricated on LiGaO2 (001) substrates by plasma assisted molecular beam epitaxy (PA-MBE). Metal-modulated-epitaxy (MME) technique was utilized to prevent formation of metal droplets during the material growth. Streaky patterns, seen in reflection high energy electron diffraction (RHEED), indicate 2-dimensional (2D) growth throughout the device. Post-deposition characterizations using scanning electron microscopy (SEM) showed smooth surfaces, while X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the epitaxial nature of the quantum well structure.