Ting-Wei Yeh

Electrical and Optical Characterization of Surface Passivation in GaAs Nanowires

We report a systematic study of carrier dynamics in Al(x)Ga(1-x)As-passivated GaAs nanowires. With passivation, the minority carrier diffusion length (L(diff)) increases from 30 to 180 nm, as measured by electron beam induced current (EBIC) mapping, and the photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns. A 48-fold enhancement in the continuous-wave PL intensity is observed on the same individual nanowire with and without the Al(x)Ga(1-x)As passivation layer, indicating a significant reduction in surface recombination. These results indicate that, in passivated nanowires, the minority carrier lifetime is not limited by twin stacking faults. From the PL lifetime and minority carrier diffusion length, we estimate the surface recombination velocity (SRV) to range from 1.7 × 10(3) to 1.1 × 10(4) cm·s(-1), and the minority carrier mobility μ is estimated to lie in the range from 10.3 to 67.5 cm(2) V(-1) s(-1) for the passivated nanowires.

InGaN/GaN Multiple Quantum Wells Grown on Nonpolar Facets of Vertical GaN Nanorod Arrays

Uniform GaN nanorod arrays are grown vertically by selective area growth on (left angle bracket 0001 right angle bracket) substrates. The GaN nanorods present six nonpolar {1⁻100} facets, which serve as growth surfaces for InGaN-based light-emitting diode quantum well active regions. Compared to growth on the polar {0001} plane, the piezoelectric fields in the multiple quantum wells (MQWs) can be eliminated when they are grown on nonpolar planes. The capability of growing ordered GaN nanorod arrays with different rod densities is demonstrated. Light emission from InGaN/GaN MQWs grown on the nonpolar facets is investigated by photoluminescence. Local emission from MQWs grown on different regions of GaN nanorods is studied by cathodoluminescence (CL). The core-shell structure of MQWs grown on GaN nanorods is investigated by cross-sectional transmission electron microscopy in both axial and radial directions. The results show that the active MQWs are predominantly grown on nonpolar planes of GaN nanorods, consistent with the observations from CL. The results suggest that GaN nanorod arrays are suitable growth templates for efficient light-emitting diodes.

Mechanism of selective area growth of GaN nanorods by pulsed mode metalorganic chemical vapor deposition

The growth mechanism for the formation of GaN nanorods using metalorganic chemical vapor deposition (MOCVD) selective area growth by pulsed source injection is proposed. The pulsed mode procedure and the kinetic model are discussed and experiments performed to support the model are described. The achievement of rod shape nanostructures grown by the pulsed mode can be attributed to two mechanisms: (1) the differences in the adsorption/desorption behavior of Ga adatoms on the c-plane (0001) and the boundary m-planes {11[overline]00}, and (2) the growth behavior of the semi-polar planes (especially the semi-polar {11[overline]00} plane).

Vertical nonpolar growth templates for light emitting diodes formed with GaN nanosheets

We demonstrate that nonpolar m-plane surfaces can be generated on uniform GaN nanosheet arrays grown vertically from the (0001)-GaN bulk material. InGaN/GaN multiple quantum wells (MQWs) grown on the facets of these nanosheets are demonstrated by cross-sectional transmission electron microscopy. Owing to the high aspect ratio of the GaN nanosheet structure, the MQWs predominantly grow on nonpolar GaN planes. The results suggest that GaN nanosheets provide a conduction path for device fabrication and also a growth template to reduce the piezoelectric field inside the active region of InGaN-based light emitting diodes.

III‐V Nanowire Array Growth by Selective Area Epitaxy

III‐V semiconductor nanowires are unique material phase due to their high aspect ratio, large surface area, and strong quantum confinement. This affords the opportunity to control charge transport and optical properties for electrical and photonic applications. Nanoscale selective area metalorganic chemical vapor deposition growth (NS‐SAG) is a promising technique to maximize control of nanowire diameter and position, which are essential for device application. In this work, InP and GaAs nanowire arrays are grown by NS‐SAG. We observe enhanced sidewall growth and array uniformity disorder in high growth rate condition. Disorder in surface morphology and array uniformity of InP nanowire array is explained by enhanced growth on the sidewall and stacking faults. We also find that AsH3 decomposition on the sidewall affects the growth behavior of GaAs nanowire arrays.

GaN nanorods for improved light emitting diode performance

GaN nanorod arrays are grown by selective area growth. The exposed nonpolar planes of the nanostructures can be utilized as a potential solution to improve the efficiency of light emitting diodes.

Nanowires in energy devices

Semiconductor nanostructures have the potential to make a positive impact on the efficiency and cost of solid state energy devices such as solar cells and light emitting diodes. In this talk I will explore the use of semiconductor nanorods for solar cells and high efficiency LEDs.

InGaN/GaN nanorod and nanosheet arrays for InGaN-based LEDs

GaN nanorods have drawn attention as possible crystalline structures on which to form InGaN active regions for LEDs, making use of the m-planes that form the sidewalls of the rods. In this paper, we describe such nanorods as well as nanosheets with increased m-plane surface area formed on c-plane sapphire. Non polar crystalline planes on nanorods and nanosheets are expected to lead to more efficient LEDs by enabling the formation of higher efficiency nonpolar active regions in three dimensional structures. The potential for increased efficiency derives from the reduction of quantum well band structure distortion that results from their formation on non-polar planes as well as the increase in the junction active area per chip by the inherent three dimensional nature of the nanostructures. Three dimensional active areas that are an order of magnitude greater than the chip area will reduce the current density at a given drive current and reduce the effects of efficiency “droop”. This increase is expected to add to efficiency improvements that derive from quantum wells on non-polar planes.