Karen Charlene Cross

Improved brightness of 380 nm GaN light emitting diodes through intentional delay of the nucleation island coalescence

Ultraviolet light emitting diodes (LEDs) have been grown using metalorganic vapor phase epitaxy, while monitoring the 550 nm reflected light intensity. During nucleation of GaN on sapphire, the transition from three-dimensional (3D) grain growth to two-dimensional (2D) coalesced growth was intentionally delayed in time by lowering the NH3 flow during the initial high temperature growth. Initially, when the reflectance signal is near zero, the GaN film is rough and composed of partly coalesced 3D grains. Eventually, the reflected light intensity recovers as the 2D morphology evolves. For 380 nm LEDs grown on 3D nucleation layers, we observe increased light output. For LEDs fabricated on GaN films with a longer recovery time an output power of 1.3 mW at 20 mA current was achieved.

Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays.

Vertically aligned InGaN/GaN nanorod light emitting diode (LED) arrays were created from planar LED structures using a new top-down fabrication technique consisting of a plasma etch followed by an anisotropic wet etch. The wet etch results in straight, smooth, well-faceted nanorods with controllable diameters and removes the plasma etch damage. 94% of the nanorod LEDs are dislocation-free and a reduced quantum confined Stark effect is observed due to reduced piezoelectric fields. Despite these advantages, the IQE of the nanorod LEDs measured by photoluminescence is comparable to the planar LED, perhaps due to inefficient thermal transport and enhanced nonradiative surface recombination.

In situ measurements of the critical thickness for strain relaxation in AlGaN∕GaN heterostructures

Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in AlxGa1−xN∕GaN heterostructures with 0.14⩽x⩽1. The surface morphology of selected films is examined by atomic force microscopy. Comparison of these measurements with critical-thickness models for brittle fracture and dislocation glide suggests that the onset of strain relaxation occurs by surface fracture for all compositions. Misfit-dislocations follow initial fracture, with slip-system selection occurring under the influence of composition-dependent changes in surface morphology.

Plan-view image contrast of dislocations in GaN

We demonstrate that when vertical threading dislocations in (0001) GaN are imaged in plan-view by transmission electron microscopy, a surface-relaxation contrast operates in addition to that due to the strain fields of dislocations passing through the specimen. We show that all three dislocation types (edge, screw, and mixed) can be detected in the same image using g=(1120) and 18° specimen tilt from [0001], allowing total densities to be assessed properly. The type of an individual dislocation can also be readily identified.

Single-mode lasing of GaN nanowire-pairs

Stable single-mode lasing operation from a pair of coupled GaN nanowires is demonstrated through optical pumping. GaN nanowires with different lengths were placed side-by-side in contact to form a coupled cavity through nanoprobe manipulation. Unlike individual nanowire lasers, which operate in a combined multiple transverse and multiple longitude mode oscillation, a coupled nanowire-pair provides a mode selection mechanism through the Vernier effect, which can strongly enhance the free spectrum range between adjacent resonant modes and generate a stable single-mode operation with a high side-mode suppression ratio.

In situ measurements of GaN nucleation layer decompostion

GaN nucleation layer (NL) decomposition was measured using optical reflectance over a wide range of pressure P, temperature T, and H2/NH3 gas mixture. The GaN NLs show measurable decomposition above 800 °C and do not significantly roughen until above 960 °C. The NL decomposition rates increase with increasing P, increasing T, and decreasing NH3 flow. An activation energy EA of 2.68 eV was measured (from 820 to 960 °C) for NL decomposition and an EA of 2.62 eV was measured (from 900 to 1075 °C) for decomposition of thick, high-T bulk GaN films. Depending on P, the pre-exponential factor A0 was four to nine times larger for NL decomposition compared to bulk GaN decomposition. The EA measured for both NL and bulk GaN decomposition in mixed H2 and NH3 flows is similar to the EA for Ga desorption, suggesting that the rate-limiting step for both NL and bulk GaN decomposition is Ga desorption.

Gold Substrate-Induced Single-Mode Lasing of GaN Nanowires.

We demonstrate a method for mode-selection by coupling a GaN nanowire laser to an underlying gold substrate. Multimode lasing of GaN nanowires is converted to single-mode behavior following placement onto a gold film. A mode-dependent loss is generated by the absorbing substrate to suppress multiple transverse-mode operation with a concomitant increase in lasing threshold of only ∼13%. This method provides greater flexibility in realizing practical single-mode nanowire lasers and offers insight into the design of metal-contacted nanoscale optoelectronics.

Indium induced step transformation during InGaN growth on GaN

The surface-step evolution of InGaN quantum-wells (QWs) was studied on GaN (0001). While the GaN template is dominated by single-monolayer steps the frequency of multiple-layer steps increases significantly when InGaN/GaN single- or multiple-QWs are grown. It is proposed that the InGaN multiple-layer step structure arises to partially accommodate the in-plane film strain which is insufficient to trigger bulk InGaN relaxation. This intrinsic multiple-layer step restructuring, when coupled with the strong piezoelectric fields present in the wurtzite group III-nitrides, could explain the enhanced carrier localization in InGaN QWs.

Polarization switching in GaN nanowire lasers

The polarization properties of a GaN nanowire laser are studied experimentally by direct analysis of the nanowires end-facet emission. Linear and elliptical light polarizations are measured at different pumping strengths. Switching between these two polarization states is also observed as the optical excitation is increased. We attribute this polarization switching to a change in the transverse modes due to their different cavity losses.

Laser diodes with 353 nm wavelength enabled by reduced-dislocation-density AlGaN templates

We fabricated optically pumped and electrically injected ultraviolet (UV) lasers on reduced-threading-dislocation-density (reduced-TDD) AlGaN templates. The overgrowth of sub-micron-wide mesas in the Al0.32Ga0.68N templates enabled a tenfold reduction in TDD, to (2–3) × 108 cm–2. Optical pumping of AlGaN hetero-structures grown on the reduced-TDD templates yielded a low lasing threshold of 34 kW/cm2 at 346 nm. Room-temperature pulsed operation of laser diodes at 353 nm was demonstrated, with a threshold of 22.5 kA/cm2. Furthermore, reduced-TDD templates have been developed across the entire range of AlGaN compositions, presenting a promising approach for extending laser diodes into the deep UV.