g-Pin Chen

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.

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.

Application of Nanosphere Lithography to LED Surface Texturing and to the Fabrication of Nanorod LED Arrays

In this study, the process of nanosphere lithography was developed and applied to LED surface texturing and nanorod device fabrication. We observed a texture-size-dependent improvement of total light output. While the increase of output optical power from the textured LEDs can be attributed to surface roughening in the GaN-air surface and to the increase of internal quantum efficiency as the strain is relaxed with the surface texturing, the size-dependent device performance is related to the interaction of generated photons with the textured surface. We further etched through the p-GaN and quantum well region to form p-i-n nanorods on the sample. By inserting a spacer to prevent p-type contact from shorting the n-GaN, we successfully demonstrated nanorod LED arrays. For such a device, a narrower radiation profile was demonstrated from the nanorod LED array as compared with that from the planar LED. The result is associated with the vertical guiding effect along the nanorod cylinder and the Bragg scattering of photons extracted from the sidewall by the rest of the rods. Furthermore, the electroluminescence spectra showed a nearly constant peak wavelength of the nanorod LED arrays, which is due to the suppression of the effect of quantum confined Stark effect.

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 LEDs 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.

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.

Characterizations of GaN-Based LEDs Encompassed With Self-Aligned Nanorod Arrays of Various Distribution Densities

We designed and fabricated GaN-based light emitting diodes encompassed with self-aligned nanorod arrays of three different distribution densities. The radiation profiles show that the device with less dense nanorod distribution has the highest optical power enhancement factor. By regarding the nanorod arrays as sidewall reflectors, the enhancement is due to the light diffraction of the laterally guided mode to radiative modes in the surface normal direction. As for the densest nanorods in our experiment, the radiation profile shows a better emission directionality as the guided modes are phase matched to the radiation modes. However, they show less optical output enhancement since some of the laterally propagated light is diffracted back to p-mesa by the first several columns of nanorods.

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.

Polarization-Dependent Sidewall Light Diffraction of LEDs Surrounded by Nanorod Arrays

The polarization behavior of the light-emitting diodes (LEDs) with nanorods surrounding the p-mesa is investigated. The nanorods were fabricated using a natural nanosphere lithography and are intended to diffract laterally propagated light. In the horizontal direction, s-polarized light is dominated since the injected carriers choose to fill up the lowest energy state in a direction parallel to the quantum-well layers. The p/s-polarized ratio starts to increase with the increase of radiated angles and eventually saturates. Since the Bragg diffraction of laterally propagated p-polarized mode by nanorods is more efficient than the s-polarized light, the p/s-ratio of the device with nanorods is higher than that without rods. The p/s-ratio of the LED with nanorods is 1.96 at 90deg, and is 1.52 when the integrating intensity between 0deg and 90deg is considered.

On-chip very low junction temperature GaN-based light emitting diodes by selective ion implantation

We propose an on-wafer heat relaxation technology by selectively ion-implanted in part of the p-type GaN to decrease the junction temperature in the LED structure. The Si dopant implantation energy and concentration are characterized to exhibit peak carrier density 1×1018 cm-3 at the depth of 137.6 nm after activation in nitrogen ambient at 750 °C for 30 minutes. The implantation schedule is designed to neutralize the selected region or to create a reverse p-n diode in the p-GaN layer, which acts as the cold zone for heat dissipation. The cold zone with lower effective carrier concentration and thus higher resistance is able to divert the current path. Therefore, the electrical power consumption through the cold zone was reduced, resulting in less optical power emission from the quantum well under the cold zone. Using the diode forward voltage method to extract junction temperature, when the injection current increases from 10 to 60 mA, the junction temperature of the ion-implanted LED increases from 34.3 °C to 42.3 °C, while that of the conventional one rises from 30.3 °C to 63.6 °C. At 100 mA, the output power of the ion-implanted device is 6.09 % higher than that of the conventional device. The slight increase of optical power is due to the increase of current density outside the cold zone region of the implanted device and reduced junction temperature. The result indicates that our approach improves thermal dissipation and meanwhile maintains the linearity of L-I curves.