Anurag Tyagi

High brightness violet InGaN/GaN light emitting diodes on semipolar (1011) bulk GaN substrates

We report the fabrication of violet InGaN/GaN light-emitting diodes (LEDs) on semipolar (1011) GaN bulk substrates. The LEDs have a dimension of 300 ?300 ?m2 and are packaged in an epoxy resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA were 20.58 mW and 33.91% respectively, with peak electroluminescence (EL) emission wavelength at 411 nm. The LEDs show minimal shift in peak EL wavelength with increasing drive current along with a high EQE.

Partial strain relaxation via misfit dislocation generation at heterointerfaces in (Al,In)GaN epitaxial layers grown on semipolar (112¯2) GaN free standing substrates

Misfit strain relaxation via misfit dislocation (MD) generation was observed in heteroepitaxially grown (Al,In)GaN layers on free-standing semipolar (112¯2) GaN substrates. Cross-section transmission electron microscope images revealed MD arrays at alloy heterointerfaces, with the MD line direction and Burgers vector parallel to [11¯00] and [112¯0], respectively. The MD structure is consistent with plastic relaxation by dislocation glide on the (0001) plane. Since (0001) is the only slip plane, the plastic relaxation is associated with tilt of the epitaxial (Al,In)GaN layers. The tilt, measured via high-resolution x-ray diffraction, can be used to quantify the relaxation.

Basal plane misfit dislocations and stress relaxation in III-nitride semipolar heteroepitaxy

This article presents a theoretical analysis of dislocation behavior and stress relaxation in semipolar III-nitride heteroepitaxy, e.g., for AlxGa1−xN and InyGa1−yN layers grown on {hh2−h−m}- or {h0h−m}-type semipolar planes of GaN substrates. We demonstrate that the shear stresses on the unique inclined basal (0001) plane do not vanish for such growth geometries. This leads to the onset of relaxation processes in semipolar III-nitride heterostructures via dislocation glide in the basal slip systems 〈1−1−20〉(0001) and to the formation of misfit dislocations (MDs) with Burgers vectors of (a/3)〈1−1−20〉-type at the semipolar heterointerface. Next we calculate the Matthews-Blakeslee critical thickness for MD formation in semipolar III-nitride layers together with the MD equilibrium spacings for complete misfit relaxation. The component of the MD Burgers vector normal to the film/substrate interface will cause a crystal lattice tilt in the epilayer with respect to the GaN substrate. The calculated magnitudes o...

Semipolar (1011) InGaN/GaN Laser Diodes on Bulk GaN Substrates

The first semipolar nitride laser diodes (LDs) have been realized on low extended defect density semipolar (1011) GaN bulk substrates. The LDs were grown by conventional metal organic chemical vapor deposition (MOCVD). Broad area lasers were fabricated and tested under pulsed conditions. Lasing was observed at a duty cycle of 0.025% with a threshold current density (Jth) of 18 kA/cm2. Stimulated emission was observed at 405.9 nm with a full width at half maximum (FWHM) of less than 0.3 nm.

Bulk GaN based violet light-emitting diodes with high efficiency at very high current density

We present experimental results on III–nitride light-emitting diodes emitting at 410 nm, grown on low-defectivity bulk GaN substrates. The epitaxial layers are optimized for high peak efficiency and maintain efficiency at very high current densities. We use a volumetric device architecture with surface roughness to maximize light extraction efficiency. We report an external quantum efficiency of 68% at 180 A cm−2. No current crowding is observed at high current density. We also demonstrate flat-line reliable operation to over 1000 h.

High power and high efficiency blue light emitting diode on freestanding semipolar (101¯1¯) bulk GaN substrate

Blue InGaN∕GaN multiple-quantum-well light emitting diodes with a peak emission wavelength of 444nm were grown on low extended defect density semipolar (101¯1¯) bulk GaN substrates by conventional metal-organic chemical vapor deposition. The calculated external quantum efficiency and output power at a drive current of 20mA under pulsed operations (10% duty cycle) were 29% and 16.21mW, respectively. The device exhibited virtually no peak electroluminescence wavelength shift with increasing drive currents, indicating a significant reduction of polarization-related internal electric fields.

Bulk GaN flip-chip violet light-emitting diodes with optimized efficiency for high-power operation

We report on violet-emitting III-nitride light-emitting diodes (LEDs) grown on bulk GaN substrates employing a flip-chip architecture. Device performance is optimized for operation at high current density and high temperature, by specific design consideration for the epitaxial layers, extraction efficiency, and electrical injection. The power conversion efficiency reaches a peak value of 84% at 85 °C and remains high at high current density, owing to low current-induced droop and low series resistance.

AlGaN-Cladding Free Green Semipolar GaN Based Laser Diode with a Lasing Wavelength of 506.4 nm

We demonstrate electrically driven InGaN based laser diodes (LDs), with a simple AlGaN-cladding-free epitaxial structure, grown on semipolar (2021) GaN substrates. The devices employed In0.06Ga0.94N waveguiding layers to provide transverse optical mode confinement. A maximum lasing wavelength of 506.4 nm was observed under pulsed operation, which is the longest reported for AlGaN-cladding-free III-nitride LDs. The threshold current density (Jth) for index-guided LDs with uncoated etched facets was 23 kA/cm2, and 19 kA/cm2 after application of high-reflectivity (HR) coatings. A characteristic temperature (T0) value of ~130 K and wavelength red-shift of ~0.05 nm/K were confirmed.

Stimulated Emission at Blue-Green (480 nm) and Green (514 nm) Wavelengths from Nonpolar (m-plane) and Semipolar (1122) InGaN Multiple Quantum Well Laser Diode Structures

Stimulated emission (SE) in the blue-green (480 nm) and green (514 nm) regime has been observed, at room temperature (RT) under optical pumping, from nonpolar m-plane (1010) and semipolar (1122) InGaN multi-quantum well (MQW) laser diode (LD) structures, respectively, grown on bulk GaN substrates. The emission intensity exhibited a clear threshold behavior with respect to the pump power. Optical anisotropy was also observed between the two perpendicular in-plane directions [1123] and [1010] for semipolar LD structures, with significantly lower pump thresholds for emission along [1123]. The SE wavelength, measured just above threshold, was blue-shifted with respect to the spontaneous emission wavelength measured just below threshold. These initial results indicate that semipolar (1122) GaN is a promising orientation for the realization of blue-green and green LDs.

Lattice Tilt and Misfit Dislocations in (1122) Semipolar GaN Heteroepitaxy

Epilayer lattice tilt has been observed by X-ray diffraction for heteroepitaxial AlGaN and InGaN films on (1122) semipolar GaN substrates. Complementary transmission electron microscopy demonstrates that epilayer tilt is a consequence of interfacial misfit dislocations with Burgers vectors a/3[1120] that glide on the (0001) basal plane [inclined ~58° to (1122)]. The dislocation lines are parallel to [1100], consistent with the anisotropy of the tilt observed in X-ray scans parallel to orthogonal in-plane directions. The dislocations had an in-plane Burgers vector component that relieved misfit strain, and a perpendicular component responsible for lattice tilt. Dislocation densities predicted by tilt agree well with TEM measurements.