Investigation of low-temperature electroluminescence of InGaN/GaN based nanorod light emitting arrays.

For InGaN/GaN based nanorod devices using a top-down etching process, the optical output power is affected by non-radiative recombination due to sidewall defects (which decrease light output efficiency) and the mitigated quantum confined Stark effect (QCSE) due to strain relaxation (which increases internal quantum efficiency). Therefore, the exploration of low-temperature optical behaviors of nanorod light emitting diodes (LEDs) will help identify the correlation between these two factors. In this work, low-temperature electroluminescent (EL) spectra of InGaN/GaN nanorod arrays were explored and compared with those of planar LEDs. The nanorod LED exhibits a much higher optical output percentage increase when the temperature decreases. The increase is mainly attributed to the increased carriers in the quantum wells for radiative recombination. Also, due to a better spatial overlap of electrons and holes in the quantum wells, the increased number of carriers can be more efficiently recombined in the nanorod device. Next, while the nanorod array shows nearly constant peak energy in the EL spectra at various injection currents at the temperature of 300 K, a blue shift has been observed at 190 K. The results suggest that with less non-radiative recombination and thus more carriers in the quantum wells, carrier screening and band filling still prevail in the partially strain relaxed nanorods. Moreover, when the temperature drops to 77 K, the blue shift of both nanorod and planar devices disappears and the optical output power decreases since there are fewer carriers in the quantum wells for radiative recombination.

High performance InGaN/GaN nanorod light emitting diode arrays fabricated by nanosphere lithography and chemical mechanical polishing processes.

We fabricated InGaN/GaN nanorod light emitting diode (LED) arrays using nanosphere lithography for nanorod formation, PECVD (plasma enhanced chemical vapor deposition) grown SiO(2) layer for sidewall passivation, and chemical mechanical polishing for uniform nanorod contact. The nano-device demonstrates a reverse current 4.77nA at -5V, an ideality factor 7.35, and an optical output intensity 6807mW/cm(2) at the injection current density 32A/cm(2) (20mA). Moreover, the investigation of the droop effect for such a nanorod LED array reveals that junction heating is responsible for the sharp decrease at the low current.

Characterizations of low-temperature electroluminescence from ZnO nanowire light-emitting arrays on the p-GaN layer.

Low-temperature electroluminescence from ZnO nanowire light-emitting arrays is reported. By inserting a thin MgO current blocking layer in between ZnO nanowire and p-GaN, high-purity UV light emission at wavelength 398 nm was obtained. As the temperature is decreased, contrary to the typical GaN-based light emitting diodes, our device shows a decrease of optical output intensity. The results are associated with various carrier tunneling processes and frozen MgO defects.

Investigation of light absorption properties and acceptance angles of nanopatterned GZO/a-Si/p(+)-Si photodiodes.

In this work, n-GZO/a:amorphous-Si(i:intrinsic)/p( + )-Si photodiodes are fabricated. We employed a nanosphere lithographic technique to obtain nanoscale patterns on either the a-Si(i) or p( + )-Si surface. As compared with the planar n-GZO/p( + )-Si diode, the devices with nanopatterned a-Si(i) and nanopatterned p( + )-Si substrates show a 32% and 36.2% enhancement of photoresponsivity. Furthermore, the acceptance angle measurement reveals that the nanostructured photodiodes have larger acceptance angles than the planar structure. It also shows that the device with the nanocone structure has a higher acceptance angle than that with the nanorod structure.

UV light emission from GZO/ZnO/GaN heterojunction diodes with carrier confinement layers.

In this work, GZO/ZnO/GaN diodes with the light emitting ZnO layer sandwiched between two SiO(2) thin films was fabricated and characterized. We observed a strong excitonic emission at the wavelength 377nm with the Mg(2+) deep level transition and oxygen vacancy induced recombination significantly suppressed. In comparison, light emission from the GZO/GaN device (without SiO(2) barriers) is mainly dominant by defect radiation. Furthermore, the device with confinement layers demonstrated a much higher UV intensity than the blue-green emission of the GZO/GaN p-n device.

Nanoparticle-coated n-ZnO/p-Si photodiodes with improved photoresponsivities and acceptance angles for potential solar cell applications.

In this work, n-ZnO/p-Si photodiodes were fabricated and characterized to explore their potential applications in solar cells. With a coating of silica nanoparticles, we observed the enhancement of photoresponsivity and acceptance angle at a wavelength between 400 and 650 nm. The 17.6% increase of the photoresponsivity over the conventional device is due to the improved optical transmission toward the semiconductor through the silica nanoparticles. Furthermore, the acceptance angle of the nanoparticle coated device is dramatically increased, which is attributed to the effect of Bragg diffraction.

Enhanced light collection of GaN light emitting devices by redirecting the lateral emission using nanorod reflectors.

We demonstrate a method of utilizing self-assembled nanorod array reflectors to collect the laterally propagating guided modes from a light emitting diode (LED). We measure an enhancement factor of 12.2% and 18.4%, respectively, from the sidewall emission of GaN-based LEDs encompassed with 10 and 20 microm thick nanorod array reflectors. Such enhancement is found to be omnidirectional due to a broken symmetry from a randomized distribution of the nanorod array placed along the periphery of the LED's mesa. These observations indicate that the use of nanorod reflectors can efficiently redirect the propagation of the laterally guided modes to the surface normal direction.

GaN nanorod light emitting diode arrays with a nearly constant electroluminescent peak wavelength.

A practical process to fabricate InGaN/GaN multiple quantum well light emitting diodes (LEDs) with a self-organized nanorod structure is demonstrated. The nanorod array is realized by using nature lithography of surface patterned silica spheres followed by dry etching. A layer of spin-on-glass (SOG), which intervening the rod spacing, serves the purpose of electric isolation to each of the parallel nanorod LED units. The electroluminescence peak wavelengths of the nanorod LEDs nearly remain as constant for an injection current level between 25mA and 100mA, which indicates that the quantum confined stark effect is suppressed in the nanorod devices. Furthermore, from the Raman light scattering analysis we identify a strain relaxation mechanism for lattice mismatch layers in the nanostructure.