Changhua Chen

Activation energies of Si donors in GaN

The electronic properties of Si donors in heteroepitaxial layers of GaN were investigated. The n‐type GaN layers were grown by metalorganic chemical vapor deposition and either intentionally doped with Si or unintentionally doped. The samples were evaluated by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy. For both types of samples the n‐type conductivity was found to be dominated by a donor with an activation energy between 12 and 17 meV. This donor is attributed to Si atoms substituting for Ga in the GaN lattice (SiGa). The range of activation energies is due to different levels of donor concentrations and acceptor compensation in our samples. The assignment of a PL signature to a donor–acceptor pair recombination involving the Si donor level as the initial state of the radiative transition yields the position of the optical Si donor level in the GaN bandgap at ∼Ec–(22±4) meV. A deeper donor level is also present in our GaN material with an activation energy of ∼3...

Hall-effect characterization of III–V nitride semiconductors for high efficiency light emitting diodes

Abstract Variable-temperature Hall-effect measurements were employed to optimize doping for GaN layers utilized in blue, blue-green and green light emitting diodes (LEDs). N-type doping was accomplished by doping with Si, Ge, and O, and the electronic properties of these donors were studied. Si and Ge, which substitute for Ga, are shallow donors with almost identical activation energies for ionization (ca. 17 and ca. 19 meV, respectively, for a donor concentration of ca. 3×10 17 cm −3 ). O substitutes for N and introduces a slightly deeper donor level into the bandgap of GaN having an activation energy of ca. 29 meV (for a donor concentration of ca. 1×10 18 cm −3 ). Mg doping was employed to achieve p-type conductivity for GaN device layers. Mg substitutes for Ga introducing a relatively deep acceptor level. For the analysis of the variable-temperature Hall-effect data, it was found important to take the coulomb interaction between ionized acceptors into account, leading to lower activation energy with increasing degree of ionization (increasing temperature). The activation energy for ionization of Mg acceptors in GaN was thus estimated to be (208±6) meV for very low acceptor concentrations. Using optimized nitride layers, LEDs with typical external quantum efficiencies of ca. 10% in the blue and blue-green, and ca. 8% in the green wavelength range were achieved. Due to optimized doping, the forward voltages for these diodes were as low as 3.2 V at 20 mA drive current.

Dislocation reduction in GaN thin films via lateral overgrowth from trenches

A technology to reduce the dislocation density in GaN thin films by lateral overgrowth from trenches (LOFT) is reported. In LOFT, a GaN thin film was grown on sapphire substrate first, then trenches were formed into the thin film by etching. GaN material was regrown laterally from the trench sidewalls to form a continuous thin film. The average surface density of threading dislocations is reduced from 8×109/cm2 in the first GaN thin film to 6×107/cm2 in the regrown GaN thin film.

Chapter 3 High-Brightness Nitride-Based Visible-Light-Emitting Diodes

Publisher Summary This chapter discusses high-brightness nitride-based visible-light-emitting diodes (LEDs). There are numerous immediate applications for the long-awaited short wavelength LEDs. These applications—for example, large-screen full-color video displays, red-amber-green traffic signal lights, and interior and exterior automotive lighting—are hindered by the lack of blue and green devices with light output performance comparable to the yellow to red aluminum gallium indium phosphide (AlGaInP) and aluminum gallium arsenide (AlGaAs) devices that have been available for 10 years. Highly efficient LEDs possess many beneficial characteristics relative to incandescent filament bulbs—namely, low power consumption and long lifetime. In addition, the maintenance costs associated with bulb replacement are greatly reduced, because an LED traffic signal head is expected to last 5 to 10 years.