T. M. Katona

Radiative and nonradiative processes in strain-free AlxGa1−xN films studied by time-resolved photoluminescence and positron annihilation techniques

Radiative and nonradiative processes in nearly strain-free AlxGa1−xN alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately −0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for x⩽0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signa...

Observation of crystallographic wing tilt in cantilever epitaxy of GaN on silicon carbide and silicon (111) substrates

We have characterized the growth of GaN on both SiC and Si (111) using the cantilever epitaxy technique. Cantilever epitaxy employs growth from periodic, parallel mesas which are formed by etching the substrate [C.I.H. Ashby et al. Appl. Phys. Lett. 77, 3233 (2000)]. GaN/AlN/Si and GaN/AlN/SiC layers were grown via metalorganic chemical vapor deposition on substrates with reactive ion etched trenches. This single step approach allows GaN to laterally overgrow the substrate trenches yielding low defect density material over the trench, while avoiding use of a mask which is commonly employed in lateral epitaxial overgrowth. Scanning electron microscopy, atomic force microscopy, and x-ray diffraction were used to characterize the resulting material. Significant threading dislocation reduction was observed in the “wing” regions (the material directly over the trenches). Contrary to previous reports, crystallographic wing tilt ranging from 0.22° to 0.77° was also observed, despite the absence of a wing/mask in...

Blue GaN-based light-emitting diodes grown by molecular-beam epitaxy with external quantum efficiency greater than 1.5%

We have grown blue (480 nm) nitride semiconductor light emitting diodes (LEDs) by plasma-assisted molecular beam epitaxy (MBE) on GaN templates. Packaged devices exhibited output powers up to 0.87 mW at 20 mA forward current. The corresponding external quantum efficiency was 1.68%. Utilizing a combination of direct current (dc) and pulsed electroluminescence measurements it has been demonstrated that at low (<20 mA) dc conditions the emission from these devices is governed by the combined effects of bandfilling and screening of electrostatic fields. However, at larger currents device heating dominates the emission properties.

Vacancy-oxygen complexes and their optical properties in AlN epitaxial films studied by positron annihilation

Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3–6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (...

Maskless lateral epitaxial overgrowth of high-aluminum-content AlxGa1−xN

We have demonstrated maskless lateral epitaxial overgrowth of Al0.96Ga0.04N on sapphire for dislocation reduction. 600 nm and 1 μm thick AlN layers were grown on sapphire via metalorganic chemical vapor deposition. Parallel, periodic trenches were then etched in the AlN and Al0.96Ga0.04N was regrown laterally from the unetched mesas. Significant threading dislocation reduction was observed for “wing” material, growing laterally, compared to “seed” material, growing vertically from the unetched mesa, as observed by atomic force microscopy, and transmission electron microscopy. Crystallographic wing tilt of ∼0.23° was measured by x-ray diffraction.

3D simulation and analysis of AlGaN/GaN ultraviolet light-emittings diodes

Compact ultraviolet light sources are currently of high interest for applications in solid-state lighting, short-range communication, and bio-chemical detection. Our nitride-based light-emitting diode (LED) includes AlGaN quantum wells with an emission wavelength of approximately 340 nm. In this paper, we analyze internal device physics by three-dimensional (3D) numerical simulation. The simulation incorporated a 3D drift-diffusion model for the carrier transport, the quantum well (QW) energy band-structure including interface polarization charges, the local QW spontaneous emission spectrum, as well as 3D raytracing for photon extraction. The simulation results showed good agreement with measurements. Internal physical mechanisms such as current crowding, carrier leakage, and carrier recombination were investigated. Nanoscale effects exhibited a strong influence on the LED performance.

Growth and fabrication of short-wavelength UV LEDs

Ultra-violet light emitting diodes with a peak wavelength of 293 nm were grown by MOCVD on AlN on sapphire. The maximum output power was 15 μW at 100 mA DC current injection for on wafer, room temperature testing. We have shown that by forming an interdigitated multi-fingered n-contact compared to a square geometry LED, the series resistance is reduced by ~ 8 - 15 Ω at 100 mA. This results in a 2 - 4 V reduction in drive voltage at 100 mA. The quantum wells exhibit a sharp electroluminescence peak at 293 nm with a 9 nm full-width at half maximum, but deep level related emission was observed at 2.56, 2.80, 3.52, and 3.82 eV. The high energy peaks, 3.52 and 3.82 eV, saturate with increasing drive current while the low energy peaks, 2.56 and 2.80 eV, increase with drive current proportional to the quantum well emission. This indicates the recombination mechanism for the low energy and high energy peaks is fundamentally different. We have also shown that forward bias leakage current in these devices is another factor limiting the quantum efficiency.

Control of crystallographic tilt in GaN grown on Si (111) by cantilever epitaxy

We have eliminated the crystallographic tilt associated with lateral growth during cantilever epitaxy by adjusting the lateral to vertical growth rate during the initial stages of growth. Cantilever epitaxy is a single growth run technique utilizing periodic, parallel mesas formed by etching the substrate. We have studied the relationship between the geometry of the “wing” region, the GaN spanning the etched trench, and the magnitude of crystallographic wing tilt. Cross-sectional scanning electron microscopy was used to characterize the geometry of the wing region, and x-ray diffraction was used to measure the magnitude of crystallographic tilt. It was found that by starting the GaN overgrowth with a slow lateral growth rate, compared to the vertical growth rate, a low tilt (⩽0.2°) was established and stabilized. The lateral growth rate can be increased during later stages of growth to assist in fast coalescence of neighboring stripes without significantly affecting the magnitude of crystallographic tilt.

Cathodoluminescence study of deep ultraviolet quantum wells grown on maskless laterally epitaxial overgrown AlGaN

We have investigated the cathodoluminescence properties of deep UV quantum wells with an emission wavelength of 287nm grown on maskless lateral epitaxial overgrown Al0.96Ga0.04N. AlN was grown on sapphire via metalorganic chemical vapor deposition. Parallel, periodic trenches were etched in the AlN and Al0.96Ga0.04N was regrown laterally from the unetched mesas. Al0.42Ga0.58N∕Al0.36Ga0.64N quantum wells were then grown on the uncoalesced stripes. Cathodoluminescence was performed on both the laterally overgrown “wings” and unetched “seed” material. Emission from quantum wells located above the wing region was observed to be more intense than emission above the seed region. Depth dependent cathodoluminescence showed that deep level recombination at 3.58eV(346nm), 2.77eV(448nm), and 2.14eV(579nm) was present throughout the n-type Al0.45Ga0.55N and the laterally overgrown unintentionally doped Al0.96Ga0.04N.

Effect of the Nucleation Layer on Stress during Cantilever Epitaxy of GaN on Si (111)

We have investigated the effect of the nucleation layer on subsequent film stress of GaN grown on Si (111) patterned with periodic mesas, using the cantilever epitaxy technique. GaN was nucleated on the unetched substrate mesa and proceeded to grow laterally over the trench region with low dislocation density material resulting in the wing region - the area over the trench. We have shown that by reducing the thickness of the AlN nucleation layer to 500 A, the uncoalesced GaN stripes are essentially crack free. We have also studied the effect of inserting a linear grade from the AlN nucleation layer to GaN. The grade increased the crack density from 4.65 × 10 -4 μm -1 with no grade to values ranging from 2.25 × 10 -2 to 1.29 × 10 -3 μm -1 . As the thickness of the grade increased the crack density approaches that obtained with no grade. X-ray diffraction was also used to characterize the nucleation layers effect on both, the structural quality and crystallographic wing tilt.