Mr. Hongwei Li Profile

Hongwei Li

Process Engineer

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Proceedings Article | 15 September 2007 Paper

The role of surface states in a-axis GaN nanowires

Alan Chin, Tai Ahn, Hongwei Li, Sreeram Vaddiraju, Christopher Bardeen, Cun-Zheng Ning, Mahendra Sunkara

Proceedings Volume 6639, 663908 (2007) https://doi.org/10.1117/12.732222

KEYWORDS: Gallium nitride, Luminescence, Picosecond phenomena, Ultraviolet radiation, Gallium, Crystals, Glasses, Edge emitting semiconductor lasers, Nanowires, Oxidation

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GaN nanowires have been the subject of intense research lately, due to the many potential ultraviolet applications and interesting properties that they possess. Because GaN has an anisotropic wurtzite crystal structure, many of its properties are dependent upon crystal orientation. For example, the photoluminescence (PL) of GaN nanowires with growth direction along the a-axis is blue-shifted relative to the PL of wires with growth direction along the c-axis. However, the origin of the difference in PL between nanowire samples of different growth directions remains unclear. To determine if surface states play a role in the dependence of GaN nanowire photoluminescence on crystal orientation, we use time-integrated photoluminescence (TIPL) and time-resolved photoluminescence (TRPL) to study the PL from GaN nanowire samples of different crystallographic orientations. We observe temporal dynamics of the blue-shifted PL feature in the a-axis GaN nanowires that is suggestive of a surface trapping process occurring, where some fraction of electron-hole pairs are prevented from recombining via the band edge emission process because carriers diffuse to the surface where they are trapped before carrier relaxation to the band edge is complete. Once a carrier is trapped and localized at a surface trap state, light emission primarily occurs only when the complementary carrier diffuses to the same surface trap. We envision that a thin oxide layer forming at the surface introduces surface traps that cause the blue emission, and that the surfaces of the a-axis GaN nanowires are more susceptible to this oxidation than the c-axis GaN nanowire surfaces.

Proceedings Article | 3 March 2006 Paper

Direction dependent homoepitaxial growth and bandgap of GaN nanowires

Mahendra Sunkara, Hongwei Li, Alan Chin

Proceedings Volume 6121, 61210I (2006) https://doi.org/10.1117/12.658050

KEYWORDS: Gallium nitride, Gallium, Plasma, Crystals, Diffusion, Nitrogen, Quartz, Hydrogen, Atmospheric plasma, Nanowires

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Here, we report a direct synthesis approach for obtaining GaN nanowires with control on growth directions: <0001> or c-direction, and <10-10> or a-direction, on amorphous substrates. The direct nitridation of Ga droplets using either dissociated ammonia or N₂/H₂ plasma resulted in GaN nanowires with <0001> growth direction; and the vapor transport of controlled (low) amounts of Ga flux in the presence of dissociated ammonia resulted in GaN nanowires with <10-10> growth direction. In both cases, the resulting GaN nanowires have diameters as small as 20 nm and lengths exceeding one hundred microns. Photoluminescence measurements showed that the bandgap of <10-10> wires blue-shifted by 50 meV from the wires with <0001> direction. Homo-epitaxial growth studies onto the pre-synthesized a-direction GaN nanowires led to belt or ribbon shaped morphologies. Homo-epitaxial growth onto c-direction wires developed micro hexagonal prism morphologies. The island growth morphologies observed on the hundred micron long, sub 30 nm size nanowires suggest that the surface transport of adatoms on c-direction wires exhibit ballistic transport or "one-dimensional" transport with mean distances over several tens of microns.

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