Hon-Way Lin

InGaN/GaN nanorod array white light-emitting diode

Conventional InGaN/GaN light-emitting diodes based on planar quantum wellstructures do not allow for efficient long-wavelength operation beyond the blue region due to a strong quantum confined Stark effect in lattice-mismatched polar InGaNquantum wells. Here we overcome the limitation by using self-assembled GaNnanorod arrays as strain-free growth templates for thick InGaN nanodisks. In combination with enhanced carrier localization and high crystalline quality, this approach allows us to realize full-color InGaN nanodisk emitters. By tailoring the numbers, positions, and thicknesses of polychromatic nanodisk ensembles embedded vertically in the GaNnanorod p - n junction, we are able to demonstrate natural white (color temperature ∼ 6000 K ) electroluminescence from InGaN/GaN nanorod arrays.

Structure and photoluminescence properties of epitaxially oriented gan nanorods grown on si(111) by plasma-assisted molecular-beam epitaxy

The authors show that vertically c-axis-aligned GaN nanorod arrays grown by plasma-assisted molecular-beam epitaxy are epitaxially oriented on Si(111) substrates and their crystal structure corresponds to a fully relaxed wurtzite lattice. At later growth stage, these GaN nanorods exhibit the tendency to coalesce into nanorod bundles. Low-temperature photoluminescence spectrum from 1-μm-long GaN nanorods consists of intense exciton lines of strain-free bulk GaN and additional lines at ∼3.21 and ∼3.42eV (Y7 and Y2). The Y7 line is attributed to the excitons trapped along the dislocations at the boundaries of coalesced GaN nanorods, while the Y2 line has its origin in the interface defects at the GaN∕Si(111) interfaces.

Single InGaN nanodisk light emitting diodes as full-color subwavelength light sources

Subwavelength electroluminescent sources with spatial, spectral, and polarization controlling capabilities are critical elements for optical imaging and lithography beyond the diffraction limit. Here, we show that the electroluminescence from single, strain-free InGaN nanodisks embedded in self-assembled GaN p - n nanorods can span the entire visible spectrum with a large linear polarization ratio ( ∼ 0.85 ) . Furthermore, this unique nanodisk-in-nanorod geometry enables the realization of the ultrasmall footprint light-emitting diodes(LEDs) to be used as subwavelength light sources. Using these nano-LEDs, we are able to demonstrate near-field, subwavelength photolithography by controlling the exposure time and light intensity from single InGaN nanodisks at chosen wavelengths.

Anion detection using ultrathin InN ion selective field effect transistors

Ultrathin (∼10nm) InN ion selective field effect transistors (ISFETs) have been demonstrated to perform ion sensing in aqueous solutions with a sensitivity of 5μA/decade and a response time smaller than 10s. The positively charged surface states on InN surfaces selectively adsorb anions, building Helmholtz voltages in solutions and modulating the drain-source current of the ISFETs. The ISFET performance is greatly enhanced by depleting carriers in the ultrathin InN channel where the film thickness is close to depth of surface electron accumulation.

Polarized photoluminescence from single GaN nanorods: Effects of optical confinement

By measuring linearly polarized photoluminescence (PL) from single, isolated gallium nitride (GaN) nanorods with the rod diameters in the subwavelength regime (30-90 nm), we present clear evidence for size dependence of polarization anisotropy. The maximum polarization ratio at room temperature (approximately 0.9 with emission and excitation light polarized parallel to the long axis of nanorod) occurs at the rod diameter of approximately 40 nm. The experimental data are compared with the recent theoretical model proposed for thick semiconductor nanowires. It is concluded that the optical confinement effects in this size regime play an important role in the observed giant polarization anisotropy. Furthermore, we have performed a temperature-dependent study of polarized PL to show the importance of internal emission anisotropy at low temperatures.

Gallium nitride nanorod arrays as low-refractive-index transparent media in the entire visible spectral region.

Vertically aligned gallium nitride (GaN) nanorod arrays grown by the catalyst-free, self-organized method based on plasma-assisted molecular-beam epitaxy are shown to behave as subwavelength optical media with low effective refractive indices. In the reflection spectra measured in the entire visible spectral region, strong reflectivity modulations are observed for all nanorod arrays, which are attributed to the effects of Fabry-Pérot microcavities formed within the nanorod arrays by the optically flat air/nanorods and nanorods/substrate interfaces. By analyzing the reflectivity interference fringes, we can quantitatively determine the refractive indices of GaN nanorod arrays as functions of light wavelength. We also propose a model for understanding the optical properties of GaN nanorod arrays in the transparent region. Using this model, good numerical fitting can be achieved for the reflectivity spectra.

Chloride ion detection by InN gated AlGaN∕GaN high electron mobility transistors

Real time chloride ion detection using InN gated AlGaN∕GaN high electron mobility transistors (HEMTs) was demonstrated. The InN thin film on the gate area of the HEMT provided fixed surface sites for reversible anion coordination. The drain current of the HEMT sensor exhibited increased a function of chloride ion concentration. The positive ions (Na+, Mg+2, and H+) in the chloride ion solutions showed no effect on the chloride ion concentration detection. The sensor was tested over a range of chloride ion concentrations from 100nMto100μM. The chloride ion HEMT sensors can be integrated with AlGaN∕GaN HEMT based pH and glucose sensors for exhaled breath condensate glucose monitoring technology. The HEMT based sensor can also be integrated into a wireless data transmission system for remote sensing applications.

Terahertz Radiation Mechanism of Native n-Type InN with Different Carrier Concentrations

The polarity and mechanism of terahertz radiation from native n-type InN excited by femtosecond optical pulses are investigated. The optical properties, electron concentrations, and crystalline quality are characterized by photoluminescence and Raman scattering spectra. The electron concentrations are estimated to be between 0.35×1019 and 3.87×1019 cm-3. The polarity of terahertz radiation field from the samples with higher electron concentrations is opposite to that from p-InAs, indicating that the dominant radiation mechanism is the drift current. However, the samples with lower electron concentrations show the same polarity as p-InAs. Under this condition, the radiation mechanism is dominated by the photo-Dember effect.

Electron-density dependence of longitudinal-optical phonon lifetime in InN studied by subpicosecond time-resolved Raman spectroscopy

Subpicosecond time-resolved Raman spectroscopy has been used to measure the lifetime of the A1(LO) and E1(LO) phonon modes in InN at T = 10 K for photoexcited electron–hole pair density ranging from 5 × 1017 to 2 × 1019 cm−3. The lifetime has been found to decrease from 2.2 ps at the lowest density to 0.25 ps at the highest density. Our experimental findings demonstrate that the carrier-density dependence of LO phonon lifetime is a universal phenomenon in polar semiconductors.