Chun-Hsiang Chang

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.

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.

Optical Properties of the Partially Strain Relaxed InGaN/GaN Light-Emitting Diodes Induced by p-Type GaN Surface Texturing

Partial strain relaxation from the light-emitting diode (LED) with surface-textured p-GaN was observed. The textured device possesses less efficiency droop and a higher current level at the efficiency maximum, as compared with the planar one. The results suggest that surface roughening affects not only the external light extraction but also the internal quantum efficiency. Furthermore, the photoluminescent (PL) measurement at low temperature reveals that the percentage increment of the optical power of the textured LED over that of the planar LED becomes lower. In addition to the effect of frozen nonradiative defect states, the PL difference is related to the strain-correlated quantum-confined Stark effect.

Effects of Strains and Defects on the Internal Quantum Efficiency of InGaN/GaN Nanorod Light Emitting Diodes

The internal quantum efficiency of GaN-based nanorod light emitting diode (LED) arrays is determined by the effects of reduced quantum confined Stark effect and sidewall-defect-related non-radiative recombination. Here we report the characterizations of light output of nanorod LED arrays with different rod etching depths. During the definition of nanorods, the effect of strain relaxation is accompanied by the formation of sidewall defects picked up from dry etching. The sample with shallower nanorods possesses fewer defects and thus a higher light output power. On the other hand, the device with longer nanorods has more relaxed strain and smaller efficiency droop. This paper indicates that a shorter nanorod etching depth is preferred for a higher light output. However, the longer nanorod structure has a less severe droop effect and a higher operating current, which may eventually lead to higher optical output if the defects can be properly suppressed.

Investigation of the strain induced optical transition energy shift of the GaN nanorod light emitting diode arrays.

Strain in the semiconductor light emitting layers has profound effect on the energy band structure and the optical properties of the light emitting diodes (LEDs). Here, we report the fabrication and characterization of GaN nanorod LED arrays. We found that the choice of nanorod passivation materials results in the variation of strain in the InGaN/GaN quantum wells, and thus the corresponding change of light emission properties. The results were further investigated by performing Raman measurement to understand the strain of nanorods with different passivation materials and by calculating the optical transition energy of the devices under the influence of strain-induced deformation potential and the piezoelectric polarization field.

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.

Polarization dependent solar cell conversion efficiency at oblique incident angles and the corresponding improvement using surface nanoparticle coating

Despite tremendous efforts on improving the solar cell conversion efficiency at normal incidence, improvement at oblique angles has not been widely addressed, not to mention the corresponding light absorption behaviors at different polarizations. Here we report the characterization of the solar cell conversion efficiency and the spectra of photoresponsivity at various tilted angles. The results show that TM (transverse magnetic) polarized light possesses higher photoresponsivity than TE (transverse electric) polarized light and the difference becomes larger with the incidence angle. To address the issue, a monolayer of silica nanoparticles on the solar cell surface was employed to improve the light absorption. Even though both TE and TM waves show a decrease in the surface reflectivity with the presence of nanoparticles, the interaction between the silica particles and the TE wave is more significant. The improvement of the conversion efficiency for obliquely incident light is explained from the refractive index difference of the nanoparticles for the TE and TM polarizations.

High-brightness light-emitting diodes

In recent years, 1D nanorods, nanocolumns, and nanowires have been synthesized, while their electrical and material properties have been studied. Among various semiconductor materials employed for nanostructures, gallium nitride (GaN) is one of the most popular choices for solid-state lighting. The sidewall areas of the nanorods that form LED arrays (see Figure 1) give them an increased effective surface. This leads to better external extraction efficiencies. The nanorods may also act as waveguides in which optical modes propagate vertically, which affects their radiation profiles. In addition, the light-emitting efficiency of GaN-based LEDs can be improved by decreasing their defect density. This can be achieved using nanorods with diameters of less than 100nm. There are two approaches to fabricating nanorod-based LED arrays. These include the bottom-up method of growing nanostructures directly on the samples and the alternative, topdown technique of growing planar epi-structures that are subsequently subject to an etching process. Fabrication of nanostructured devices using the latter approach presents several technical and economic challenges. The commercial viability of nanostructures depends on the ability to obtain an affordable nanopattern definition covering large chip sizes. In addition, the nanorod-sidewall damage needs to be repaired. Otherwise, this will lead to sidewall leakage currents and nonradiative recombination, which will seriously degrade device performance. Moreover, since each nanorod forms a vertically stacked p-i-n multilayer structure (see Figure 2)—where the intrinsic (i) part is usually composed of multiple quantum wells—a suitable planarization layer is required to prevent the p-type contact metal from shorting the underlying n-type semiconductor. Our group has developed techniques to address these issues. We established nanosphere lithography, a technique that Figure 1. Scanning-electron-microscopy image of the nanorod array.

Low-temperature electroluminescent behaviors of InGaN/GaN-based nanorod light emitting diode arrays

For InGaN/GaN based nanorod devices using top-down etching process, the optical output power is affected by non-radiative recombination due to sidewall defects (which decrease light output efficiency) and 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 those two factors. In this work, low-temperature EL spectra of InGaN/GaN nanorod arrays was 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 and a better spatial overlap of electrons and holes in the quantum wells for radiative recombination. Next, while the nanorod array shows nearly constant peak energy with increasing injection currents at the temperature of 300K, the blue shift has been observed at 190K. The results suggest that with 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 77K, the blue shift of both nanorod and planar devices disappears and the optical output power decreases since there are few carriers in the quantum wells for radiative recombination.