Anisotropic Etching of InGaN Thin Films with Photoelectrochemical Etching to Form Quantum Dots.

Traditional methods for synthesizing InGaN quantum dots (QDs), such as the Stranski-Krastanov growth, often result in QD ensembles with low density and non-uniform size distribution. To overcome these challenges, forming QDs using photoelectrochemical (PEC) etching with coherent light has been developed. Anisotropic etching of InGaN thin films is demonstrated here with PEC etching. InGaN films are etched in dilute H2SO4 and exposed to a pulsed 445 nm laser with a 100 mW/cm2 average power density. Two potentials (0.4 V or 0.9 V) measured with respect to an AgCl|Ag reference electrode are applied during PEC etching, resulting in different QDs. Atomic force microscope images show that while the QD density and sizes are similar for both applied potentials, the heights are more uniform and match the initial InGaN thickness at the lower applied potential. Schrodinger-Poisson simulations show that polarization-induced fields in the thin InGaN layer prevent positively charged carriers (holes) from arriving at the c-plane surface. These fields are mitigated in the less polar planes resulting in high etch selectivity for the different planes. The higher applied potential overcomes the polarization fields and breaks the anisotropic etching.

Ultra-Broadband Optical Gain in III-Nitride Digital Alloys.

A novel III-Nitride digital alloy (DA) with ultra-broadband optical gain is proposed. Numerical analysis shows a 50-period InN/GaN DA yields minibands that are densely quantized by numerous confined states. Interband transitions between the conduction and valence minibands create ultra-broadband optical gain spectra with bandwidths up to ~1 µm that can be tuned from the red to infrared. In addition, the ultra-broadband optical gain, bandwidth, and spectral coverage of the III-Nitride DA is very sensitive to layer thickness and other structural design parameters. This study shows the promising potential of the III-Nitride DAs with tunable ultra-broadband interband optical gain for use in semiconductor optical amplifiers and future III-Nitride photonic integration applications.

Spatial mapping of efficiency of GaN/InGaN nanowire array solar cells using scanning photocurrent microscopy.

GaN-InGaN core-shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective absorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices.

Three-dimensional mapping of quantum wells in a GaN/InGaN core-shell nanowire light-emitting diode array.

Correlated atom probe tomography, cross-sectional scanning transmission electron microscopy, and cathodoluminescence spectroscopy are used to analyze InGaN/GaN multiquantum wells (QWs) in nanowire array light-emitting diodes (LEDs). Tomographic analysis of the In distribution, interface morphology, and dopant clustering reveals material quality comparable to that of planar LED QWs. The position-dependent CL emission wavelength of the nonpolar side-facet QWs and semipolar top QWs is correlated with In composition.

III-nitride core­shell nanowire arrayed solar cells.

A solar cell based on a hybrid nanowire­film architecture consisting of a vertically aligned array of InGaN/GaN multi-quantum well core­shell nanowires which are electrically connected by a coalesced p-InGaN canopy layer is demonstrated. This unique hybrid structure allows for standard planar device processing, solving a key challenge with nanowire device integration, while enabling various advantages by the nanowire absorbing region such as higher indium composition InGaN layers by elastic strain relief, more efficient carrier collection in thinner layers, and enhanced light trapping from nano-scale optical index changes. This hybrid structure is fabricated into working solar cells exhibiting photoresponse out to 2.1 eV and short-circuit current densities of ~1 mA cm(-2) under 1 sun AM1.5G. This proof-of-concept nanowire-based device demonstrates a route forward for high-efficiency III-nitride solar cells.

High-efficiency, microscale GaN light-emitting diodes and their thermal properties on unusual substrates.

A method for forming efficient, ultrathin GaN light-emitting diodes (LEDs) and for their assembly onto foreign substances is reported. The LEDs have lateral dimensions ranging from ~1 mm × 1 mm to ~25 µm × 25 µm. Quantitative experimental and theoretical studies show the benefits of small device geometry on thermal management, for both continuous and pulsed-mode operation, the latter of which suggests the potential use of these technologies in bio-integrated contexts.