38 research outputs found
Fingerprints of carbon defects in vibrational spectra of gallium nitride (GaN) consider-ing the isotope effect
This work examines the carbon defects associated with recently reported and
novel peaks of infrared (IR) absorption and Raman scattering appearing in GaN
crystals at carbon () doping in the range of concentrations from
to . 14 unique vibrational modes of defects
are observed in GaN samples grown by hydride vapor phase epitaxy (HVPE) and
then compared with defect properties predicted from first-principles
calculations. The vibrational frequency shift in two enriched samples
related to the effect of the isotope mass indicates six distinct configurations
of the carbon-containing point defects. The effect of the isotope replacement
is well reproduced by the density functional theory (DFT) calculations.
Specific attention is paid to the most pronounced defects, namely tri-carbon
complexes() and carbon substituting for nitrogen . The position
of the transition level (+/0) in the bandgap found for defects by
DFT at 1.1 eV above the valence band maximum, suggest that
provides compensation of . defects are observed to be
prominent, yet have high formation energies in DFT calculations. Regarding
defects, it is shown that the host Ga and N atoms are involved in the
defect's delocalized vibrations and significantly affect the isotopic frequency
shift. Much more faint vibrational modes are found from di-atomic carbon-carbon
and carbon-hydrogen (C-H) complexes. Also, we note changes of vibrational mode
intensities of , , C-H, and defects in the IR
absorption spectra upon irradiation in the defect-related UV/visible absorption
range. Finally, it is demonstrated that the resonant enhancement of the Raman
process in the range of defect absorption above 2.5 eV enables the detection of
defects at carbon doping concentrations as low as
AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy
We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 ”m crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 109 cm-2. However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %)
Shallow donor and DX states of Si in AlN
In unintentionally Si-doped AlN, the electron paramagnetic resonance (EPR) spectrum of the Si shallow donor (g=1.9905) was observed in darkness at room temperature. The temperature dependence of the EPR signal suggests that Si in AlN is a DX center with the DX- state lying at similar to 78 meV below the neutral shallow donor state. With such relatively small thermal activation energy, Si is expected to behave as a shallow dopant in AlN at normal device operating temperatures.Original Publication:Nguyen Son Tien, M Bickermann and Erik Janzén, Shallow donor and DX states of Si in AlN, 2011, APPLIED PHYSICS LETTERS, (98), 9, 092104.http://dx.doi.org/10.1063/1.3559914Copyright: American Institute of Physicshttp://www.aip.org
Defects at nitrogen site in electron-irradiated AlN
In high resistance AlN irradiated with 2 MeV electrons, an electron paramagnetic resonance (EPR) spectrum, labeled EI-1, with an electron spin S = 1/2 and a clear hyperfine (hf) structure was observed. The hf structure was shown to be due the interaction between the electron spin and the nuclear spins of four 27A nuclei with the hf splitting varying between ⌠6.0 and ⌠7.2âmT. Comparing the hf data obtained from EPR and ab initio supercell calculations we suggest the EI-1 defect to be the best candidate for the neutral nitrogen vacancy in AlN