T. Paskova

Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

We report on the emission properties of nonpolar a -plane GaN layers grown on r -plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resol ...

Green light emitting diodes on a-plane GaN bulk substrates:

We report the development of 520–540nm green light emitting diodes (LEDs) grown along the nonpolar a axis of GaN. GaInN∕GaN-based quantum well structures were grown in homoepitaxy on both, a-plane bulk GaN and a-plane GaN on r-plane sapphire. LEDs on GaN show higher, virtually dislocation-free crystalline quality and three times higher light output power when compared to those on r-plane sapphire. Both structures show a much smaller wavelength blue shift for increasing current density (<10nm for 0.1to12.7A∕cm2) than conventional LEDs grown along the polar c axis.

Wavelength-stable cyan and green light emitting diodes on nonpolar m-plane GaN bulk substrates

We report the development of 480 nm cyan and 520 nm green light emitting diodes (LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to 38u2002A/cm2. LEDs have been obtained in GaInN-based homoepitaxy on nonpolar m-plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells (2×108 to ∼1010u2002cm2) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.

Contribution of free-electron recombination to the luminescence spectra of thick GaN films grown by hydride vapor phase epitaxy

Emission spectra of thick unintentionally doped GaN layers grown on sapphire by hydride vapor phase epitaxy have been studied. In addition to sharp exciton emission lines, a broad asymmetric band extending to energies above the band gap is observed in the near band-gap regions of the photoluminescence and cathodoluminescence spectra. We explain this feature as a free-electron recombination band. Using spatially resolved cathodoluminescence we were able to separately record the spectra of lightly doped regions which contain only sharp bound exciton, the spectra of highly doped regions which show the broadband only, as well as the spectra of the areas containing surface defects which show both spectral features simultaneously. Using a model in which the electrons are degenerate and the holes are localized together with the results of Hall-effect measurements we can simulate the spectral shape of the broadband. Raman scattering spectroscopy confirms the coexistence of lightly doped (∼1×1017u200acm−3) and heavily...

Defect distribution in a-plane GaN on Al2O3

The authors studied the structural and point defect distributions of hydride vapor phase epitaxial GaN film grown in the [11−20] a direction on (1−102) r-plane sapphire with metal-organic vapor phase deposited a-GaN template using transmission electron microscopy, secondary ion mass spectrometry, and positron annihilation spectroscopy. Grown-in extended and point defects show constant behavior as a function of thickness, contrary to the strong nonuniform defect distribution observed in GaN grown along the [0001] direction. The observed differences are explained by orientation-dependent and kinetics related defect incorporation.

Direct observation of large-scale nonuniformities in hydride vapor-phase epitaxy-grown gallium nitride by cathodoluminescence

Hydride vapor phase epitaxial GaN films grown on sapphire without a buffer are found to contain large-scale regions with high electron concentration located close to the interface. These regions are composed of individual columns forming an irregular but quasicontinuous layer, while the rest of the film has a much lower carrier concentration. The highly doped regions are easily visualized using cathodoluminescence. The coexistence of regions with low and high electron concentration allows us to explain the concurrent evidence of high film quality in photoluminescence, Raman spectroscopy and x-ray diffraction, and a high electron concentration measured in transport studies.

Highly Polarized Green Light Emitting Diode in m-Axis GaInN/GaN

Linearly polarized light emission is analyzed in nonpolar light-emitting diodes (LEDs) covering the blue to green spectral range. In photoluminescence, m-plane GaInN/GaN structures reach a polarization ratio from 0.70 at 460 nm to 0.89 at 515 nm peak wavelength. For a-plane structures, the polarization ratio is 0.53 at 400 nm and 0.60 at 480–510 nm. In electroluminescence the polarization ratio is 0.77 at 505 nm in 350×350 µm2m-plane devices at 20 mA. Such a device should allow 44% power saving compared with nonpolarized c-plane LEDs combined with a polarizing filter, as commonly used in LED-backlit liquid crystal displays.

Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy

The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.

Time-resolved spectroscopy of strained GaN/AlN/6H–SiC heterostructures grown by metalorganic chemical vapor deposition

Temperature-dependent time-resolved photoluminescence measurements were performed on GaN film/AlN buffer/6H–SiC substrate heterostructures grown by metalorganic chemical vapor deposition. The overlying GaN layers were under tension, as estimated from the free A exciton (FEA) position. The recombination lifetimes were determined for the FEA and for the neutral-donor-bound exciton (D0X). We observed that the recombination lifetime for the FEA has the same value of 40–50 ps in all the layers, whereas the recombination time for the D0X varies for different samples. We observed that the recombination lifetimes for D0X have a clear dependence on the position of FEA, i.e., the recombination lifetime increases with decreasing strain in the layers. We discuss the results in term of the hole states involved in the donor-bound exciton recombination.

Microscopic emission properties of nonpolar a-plane GaN grown by HVPE

We have studied the emission distributions in nonpolar α-plane GaN thick films grown by HVPE using different nucleation schemes. The emission spectra show in addition to the near band edge emission band, also defect related bands due to different structural defects being enhanced/reduced to different extent in samples grown on different templates. Spatially resolved cathodoluminescence imaging reveals the in-plane distributions of the respective emission bands, which allows us to correlate the emissions with particular stacking fault structural defects independently revealed by plan-view transmission electron microscopy. In addition, emission distributions were visualized in vicinity of largescale defects like surface triangle pits, depressions and cracks attributed to prevailing defect formation and/or impurity incorporation.