John F. Kaeding

292 nm AlGaN Single-Quantum Well Light Emitting Diodes Grown on Transparent AlN Base

We report the use of a novel transparent AlN base layer to achieve sub-300 nm electroluminescence from an AlGaN p-n junction single quantum well light emitting diode grown on sapphire by metal organic chemical vapor deposition. For unpackaged devices tested on wafer at a continuous current of 120 mA, an optical power of 2.4 µW was achieved. Peak emission was 292 nm, with very little secondary wavelength emission.

Effect of nitridation on polarity, microstructure, and morphology of AlN films

The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was ∼1×108 and 2×1010 cm−2 for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {1102} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost f...

Relation between Al vacancies and deep emission bands in AlN epitaxial films grown by NH3-source molecular beam epitaxy

Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1eV to the near-band-edge emissions at 11K of AlN epilayers grown by NH3-source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8eV bands increased remarkably with lowering supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48meV.

Impact of strain on free-exciton resonance energies in wurtzite AlN

The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ϵ=7.87, ϵ⊥=7.33, and ϵ∥=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1−xN is also presented.

Photoelectrochemical Properties of Nonpolar and Semipolar GaN

The photoelectrochemical and electrical properties of nonpolar (1120)-oriented and semipolar (1122)-oriented GaN were compared with those of (0001)-oriented GaN. Flatband potentials were obtained in the order of (1120)<(0001)<(1122). The highest photocurrent at a zero bias had been expected for the (1120) sample considering the flatband potential, but the photocurrent of the (1120) sample was the lowest among the three. This could have been due to the electric properties of the (1120) sample used. The surface morphology changes indeed by the photoelectrochemical reactions are also discussed.

Effect of Substrate Miscut on the Direct Growth of Semipolar (10-1-1) GaN on (100) MgAl2O4 by Metalorganic Chemical Vapor Deposition

We report on the the direct growth of semipolar (10-1-1) GaN templates on (100) MgAl2O4 substrates miscut in the direction. (10-1-1) GaN deposited on nominally on-axis substrates did not coalesce. An intentional substrate miscut broke the four-fold symmetry of the substrate surface and resulted in coalesced layers. The minimum full-width at half maximum (FWHM) of the on-axis (10-1-1) X-ray rocking curve was 0.4° for a 1.5° miscut. The FWHM of the off-axis (000-2) rocking curve remained constant for miscuts <3.0°. 93% and 99% of the volume of the samples were aligned in a single epitaxial variant for 1.5° and 3.0° substrate miscuts, respectively.

Realization of high hole concentrations in Mg doped semipolar (101¯1¯) GaN

The authors report on the growth of Mg doped semipolar (101¯1¯) GaN layers on vicinal (100) MgAl2O4 substrates miscut in the ⟨011⟩ direction by metal-organic chemical vapor deposition. A maximum hole concentration of 2.4×1018cm−3 and a maximum mobility of 8cm2V−1s−1, respectively, were achieved following a postgrowth thermal annealing step. Although the hole concentration decreased for Mg concentrations greater than 3.3×1019cm−3, significant hole concentrations, combined with the reduction in internal polarization fields, make semipolar GaN layers suitable for the fabrication of high brightness optoelectronic devices.

Photoluminescence and positron annihilation studies on Mg-doped nitrogen-polarity semipolar (101¯1¯) GaN heteroepitaxial layers grown by metalorganic vapor phase epitaxy

The influences of enhanced stacking fault (SF) formation, which is peculiar to nitrogen-(N-) polarity growth and lattice-mismatched semipolar heteroepitaxy, on the electrical properties of (101¯1¯) Mg-doped GaN (GaN:Mg) epilayers were investigated. Although the residual donor concentration was higher than (0001) GaN because of N-polar growth, comparatively low Mg doping (3×1019 cm−3) gave a hole concentration approximately 1.5×1018 cm−3, which was an order of magnitude higher than (0001) GaN:Mg. As the acceptor ionization energy estimated from low temperature photoluminescence was quite similar for (101¯1¯) and (0001) GaN:Mg, the high Mg activation seems to result with the aid of high density SFs. Because the Doppler broadening S parameter for the positron annihilation measurement, which reflects the concentration or size of negatively charged cation vacancies, of (101¯1¯) GaN:Mg was smaller than (0001) case, (101¯1¯) orientation is well suited to Mg-doping.