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Lee, Stephen Roger

Publications and source records attributed to Lee, Stephen Roger.

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Semi-polar GaN Materials Technology for High IQE Green LEDs

The goal of this NETL funded program was to improve the IQE in green (and longer wavelength) nitride- based LEDs structures by using semi-polar GaN planar orientations for InGaN multiple quantum well (MQW) growth. These semi-polar orientations have the advantage of significantly reducing the piezoelectric fields that distort the QW band structure and decrease electron-hole overlap. In addition, semipolar surfaces potentially provide a more open surface bonding environment for indium incorporation, thus enabling higher indium concentrations in the InGaN MQW. The goal of the proposed work was to select the optimal semi-polar orientation and explore wafer miscuts around this orientation that produced the highest quantum efficiency LEDs. At the end of this program we had hoped to have MQWs active regions at 540 nm with an IQE of 50% and an EQE of 40%, which would be approximately twice the estimated current state-of-the-art.

Koleske, Daniel D.↗
DOE OSTI2012

III-nitride core-shell nanowire arrayed solar cells

A solar cell based on a III-nitride hybrid nanowire-film architecture is demonstrated. It consists of a vertically-aligned array of InGaN/GaN multi-quantum well core-shell nanowires, electrically connected by a coalesced p-type InGaN canopy layer. This hybrid structure allows for standard planar device processing, solving an important challenge with nanowire device integration. It also enables various advantages such as higher indium composition InGaN layers via elastic strain relief, efficient carrier collection through thin layers, and enhanced light trapping. This proof-of-concept nanowire-based device presents a path forward for high-efficiency III-nitride solar cells. Fabricated III-nitride nanowire solar cells exhibit a photoresponse out to 2.1eV and short circuit current density of ~1 mA/cm 2 (1 sun AM1.5G).

Wierer Jr., Jonathan J.↗
DOE OSTI2012

Nanofabrication of tunable nanowire lasers via electron and ion-beam based techniques

GaN nanowires were created from planar GaN epilayers using a top-down fabrication technique consisting of a dry etch followed by an anisotropic wet etch. The GaN nanowires are m-plane bounded with tightly-controlled dimensions. Single-mode lasing was achieved with GaN nanowire with 130 nm diameter and 5 μm length. Numerical simulations based on a multimode laser theory indicate that the suppression of transverse and longitudinal side-modes is caused by strong mode competition and narrow gain bandwidth. For multiple mode GaN nanowire lasers with large diameter, we demonstrated that coupled nanowire pairs provide a mode selection mechanism through Vernier effect and become single mode lasers. Moreover, transverse-mode suppression and single-mode lasing is demonstrated in GaN nanowires in contact with gold substrates, which introduces an attenuation effect to the cavity modes. The finite-difference time domain simulations confirm the absorption effect, which give rise to a mode selection mechanism for realizing single mode operation.

Li, Qiming↗
DOE OSTI2011

Science-based Solutions to Achieve High-performance Deep-UV Laser Diodes (Final LDRD Report)

We present the results of a three year LDRD project that has focused on overcoming major materials roadblocks to achieving AlGaN-based deep-UV laser diodes. We describe our growth approach to achieving AlGaN templates with greater than ten times reduction of threading dislocations which resulted in greater than seven times enhancement of AlGaN quantum well photoluminescence and 15 times increase in electroluminescence from LED test structures. We describe the application of deep-level optical spectroscopy to AlGaN epilayers to quantify deep level energies and densities and further correlate defect properties with AlGaN luminescence efficiency. We further review our development of p-type short period superlattice structures as an approach to mitigate the high acceptor activation energies in AlGaN alloys. Finally, we describe our laser diode fabrication process, highlighting the development of highly vertical and smooth etched laser facets, as well as characterization of resulting laser heterostructures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗