Xiaobiao Han

Vertical-conducting InGaN/GaN multiple quantum wells LEDs with AlN/GaN distributed Bragg reflectors on Si(111) substrate

In this study, crack-free InGaN/GaN multiple quantum well (MQW) LEDs with five-pair AlN/GaN distributed Bragg reflectors (DBRs) were grown on a Si(111) substrate using a linear compositionally graded AlGaN layer to compensate for tensile stresses. DBR-based LEDs exhibited higher light efficiency and better crystalline quality than non-DBR-based LEDs. Vertical-conducting DBR-based LEDs were realized using through-hole structure. Through-holes were formed from the n-GaN layer to the Si substrate and filled with metals, which connected the n-GaN layer and the Si substrate. Insulating AlN, high-resistivity AlGaN layers, and the large band offset at the AlN/Si interface were all shorted by the metal filled into the through-holes. Compared to DBR-based LED without through-holes, DBR-based LEDs with through-holes exhibited significantly better electrical and optical properties.

Improving the Quality of GaN on Si(111) Substrate with a Medium-Temperature/High-Temperature Bilayer AlN Buffer

A medium-temperature/high-temperature (MT/HT) bilayer AlN buffer was introduced for GaN grown on Si(111) by metal organic chemical vapor deposition. The properties of the GaN films with a MT/HT bilayer AlN buffer and those with a single-layer HT-AlN buffer were compared and the influence of the growth temperature of the MT-AlN layer was investigated. With a MT-AlN layer grown in the temperature range from 800 to 1000 °C, the crystalline qualities of the subsequent HT-AlN layer and the GaN film were improved. According to the X-ray diffraction results and the transmission electron microscopy images, the dislocation density in GaN film was reduced with a MT/HT bilayer AlN buffer as compared to those with a single-layer HT-AlN buffer. Moreover, photoluminescence and Raman spectra exhibit enhanced optical properties and less tensile stresses of the GaN film. Better surface morphology of GaN was also obtained with a MT/HT bilayer AlN buffer.

Influence of adatom migration on wrinkling morphologies of AlGaN/GaN micro-pyramids grown by selective MOVPE*

GaN micro-pyramids with AlGaN capping layer are grown by selective metal–organic–vapor phase epitaxy (MOVPE). Compared with bare GaN micro-pyramids, AlGaN/GaN micro-pyramids show wrinkling morphologies at the bottom of the structure. The formation of those special morphologies is associated with the spontaneously formed AlGaN polycrystalline particles on the dielectric mask, owing to the much higher bond energy of Al–N than that of Ga–N. When the sizes of the polycrystalline particles are larger than 50 nm, the uniform source supply behavior is disturbed, thereby leading to unsymmetrical surface morphology. Analysis reveals that the scale of surface wrinkling is related to the migration length of Ga adatoms along the AlGaN facet. The migration properties of Al and Ga further affect the distribution of Al composition along the sidewalls, characterized by the μ-PL measurement.

Migration characterization of Ga and In adatoms on dielectric surface in selective MOVPE

Migration characterizations of Ga and In adatoms on the dielectric surface in selective metal organic vapor phase epitaxy (MOVPE) were investigated. In the typical MOVPE environment, the selectivity of growth is preserved for GaN, and the growth rate of GaN micro-pyramids is sensitive to the period of the patterned SiO2 mask. A surface migration induced model was adopted to figure out the effective migration length of Ga adatoms on the dielectric surface. Different from the growth of GaN, the selective area growth of InGaN on the patterned template would induce the deposition of InGaN polycrystalline particles on the patterned SiO2 mask with a long period. It was demonstrated with a scanning electron microscope and energy dispersive spectroscopy that the In adatoms exhibit a shorter migration length on the dielectric surface.

Enhancement of emission of InGaN/GaN multiple-quantum-well nanorods by coupling to Au-nanoparticle plasmons

We demonstrate the enhancement of emission of InGaN/GaN multiple-quantum-well nanorods by nearly a factor of 2 by coupling them to localized surface plasmons of Au nano-particles (NPs). The Au NPs are fabricated in situ on the nanorods using a Ni/SiO2/Au/SiNx compound functional layer. This layer serves as a combination dry-etch mask for fabricating the nanorods and the Au NPs, as well as providing isolation necessary to prevent fluorescence quenching. Time-resolved photoluminescence measurements confirm that emission enhancement originates from the coupling.We demonstrate the enhancement of emission of InGaN/GaN multiple-quantum-well nanorods by nearly a factor of 2 by coupling them to localized surface plasmons of Au nano-particles (NPs). The Au NPs are fabricated in situ on the nanorods using a Ni/SiO2/Au/SiNx compound functional layer. This layer serves as a combination dry-etch mask for fabricating the nanorods and the Au NPs, as well as providing isolation necessary to prevent fluorescence quenching. Time-resolved photoluminescence measurements confirm that emission enhancement originates from the coupling.

Influence of an integrated quasi-reference electrode on the stability of all-solid-state AlGaN/GaN based pH sensors

An all-solid-state AlGaN/GaN based ion-sensitive heterostructure field effect transistor (ISHFET) pH sensor was fabricated by integrating a noble metal (Au) quasi-reference electrode to improve the device stability when measuring the pH value of a small aqueous volume. In this paper, the influence of the size of the quasi-reference electrode against the stability of the pH readings was investigated. Through optimizing the size of the integrated quasi-reference electrode, the all-solid-state ISHFET pH sensor can sustain stable pH measurements for aqueous solutions of micro-litre size. A sensitivity of 55 mV/pH was achieved by the pH sensor at room temperature. Thus, the device may have potential uses in biomedical applications which require small volume pH measurements.An all-solid-state AlGaN/GaN based ion-sensitive heterostructure field effect transistor (ISHFET) pH sensor was fabricated by integrating a noble metal (Au) quasi-reference electrode to improve the device stability when measuring the pH value of a small aqueous volume. In this paper, the influence of the size of the quasi-reference electrode against the stability of the pH readings was investigated. Through optimizing the size of the integrated quasi-reference electrode, the all-solid-state ISHFET pH sensor can sustain stable pH measurements for aqueous solutions of micro-litre size. A sensitivity of 55 mV/pH was achieved by the pH sensor at room temperature. Thus, the device may have potential uses in biomedical applications which require small volume pH measurements.

Dual-Color InGaN/GaN Pyramidal Micro Light-Emitting Diode Selectively Grown on SiO Masked Si Substrate

Dual-color InGaN/GaN pyramidal micro light-emitting diode ( μ-LED) was directly grown on SiO2 masked Si substrate via selective area growth technology. An electrically driven dual-color μ-LED with typical rectifying behavior and a turn-on voltage of about 3 V was demonstrated. Two emission peaks locating at 440 and 490 nm were observed by micro-photoluminescence. Using catholuminescence measurement, it was found that the blue emission originated from the apex of micro pyramid top, and green emission was mainly from the lower part of side facets. By controlling the injection current, electroluminescence emission color of the μ-LED can be tuned continuously from green to blue range.